Thanks to alice_bluu for letting us use this guide.
So lets talk about stats
First things first, viewing your stats.
Hold ALT to view additional information when a tool tip is active. This includes dice, expanded definitions for any acronyms, etc.
Hold SHIFT to view the next tier of an ability (if available) when an ability tool tip is active.
Dice values have been replaced with a damage range that includes all self contained bonuses (previously shown as mods or 1d4+5 etc). No back-end math has changed - everything is still based on dice rolls (which can be viewed on the tooltip by holding ALT).
Colors of items on tooltips...
Blue = player stats.
Light Green = augments.
Green = reagents.
When viewing the requirements section blue = requirement met, red = requirement not met.
Note - stats that do not operate as a % are...
- Haste.
- Safe Fall.
- Hit.
- Positional (when its contributing to Hit).
- Armour Class.
- Durability.
- Armour Weight.
General Stats
Movement - Determines how fast you move out of combat stance.
Combat Movement - Determines how fast you move in combat stance. It is only active while in combat stance.
Haste - This determines how long your cooldowns are. Does NOT effect alchemy cooldowns! (1+haste/100) i.e. +50 haste on a 60s cd is 60s / (1+0.5) = 40s.
Safe fall - Reduces the damage you take when you fall off of things. (never trust a rock!).
Health Regeneration - The rate at which your health regenerates. Is significantly reduced in combat.
Stamina Regeneration - The rate at which your stamina regenerates. Does apply in combat but is significantly diminished.
Healing - Gives a bonus to your outgoing heal abilities. Does not affect potions or incoming heals.

Combat - Defense
Resilience - Saving throw. Rolls for a chance at not falling in combat upon reaching 0 health. Resilience also reduces the damage bonus of incoming heavy and critical hits. Reminder that resilience only works in combat stance!
Avoid - Chance to avoid an attack. Negating all damage and effects. (think dodge/evade).
Block - Chance to block an attack. Block mitigates incoming damage based on a dice roll from your equipped 2H weapon or shield.
Parry - Chance to parry an attack. Parry provides a fixed 15% damage mitigation, the incoming attack suffers a disadvantage, and you have an opportunity to counter-attack with a riposte.
Riposte - Chance to counterattack after a successful parry. 2x auto attack damage and 2x threat.

Combat - Offense
Hit - Increases chance to land a heavy or critical hit and reduces the chance of getting a glance or miss.
Penetration - Ignores some of the enemies' armor dealing more direct damage to health.
Positional - Increases your weapon positional bonus. (formerly known as flanking).
Most weapons have a Primary and Secondary positional, typically denoted by a reduced base positional value. The Primary direction gets a bonus to the positional stat, but the Secondary will only receive the base value of the positional stat.
Let me break it down into a formula.
Positional bonus stat (this would be the stat you get from the bonus like hit or 1h dmg) = B.
Your personal positional stat value which is shown on your stat sheet. = S.
The % value of the positional bonus, primary or secondary = P.
The formula would go as follows B+S*P = X.
Lets use Undones example - a Crossbow with +5DMG Side Positional (Primary) and +15 Hit Rear Positional (Secondary). With 25 positional stat on the player.
The primary position from the side will have 5+25*1.25 = 36.25 DMG.
Whereas the secondary position from the rear will have 15+25*1 = 40 HIT.
I have inserted an image so you can see what positional stats look like on a weapon.

Combat - Damage
1 Hand Weapons - Increase your damage with 1h weapons.
2 Hand Weapons - Increase your damage with 2h weapons.
Ranged Weapons - Increase your damage with bows and crossbows.
Mental - Increase your damage with mental abilities.
Chemical - Increase your damage with chemical abilities.
Ember - Increase your damage with alchemy abilities.
Resists
Damage
Physical - reduce amount of physical damage taken.
Mental - reduce the amount of mental damage taken.
Chemical - reduce amount of chemical damage taken.
Ember - reduce the amount of ember damage taken.
Debuffs
Physical - reduce duration of physical debuffs.
Mental - reduce the duration of mental debuffs.
Chemical - reduce duration of chemical debuffs.
Ember - reduce the duration of ember debuffs.
Movement - reduce duration of movement debuffs.
Control
Confuse - reduce the duration of confuse effects.
Daze - reduce the duration of daze effects.
Stun - reduce the duration of stun effects.

Combat Log
You can view tool tips for things in the combat log by simply mousing over them in the log.ABS - absorb. damage absorbed by armor from all roll sources. damage absorbed by armor contributes to the armor total % durability.
THR - threat generated.
ADV - the attack had an advantage.
DIS - the attack had a disadvantage.
ADV - the attack had an advantage that was cancelled out by a disadvantage. (i need to double check this one as far as what it looks like in the combat log).
RES - the application was resisted.
PRES - the application was partially resisted.
DIM - the application was impacted by diminishing returns.
AOT - the value done by over time effects.
Miss - attack missed and did not hit. effect does not take hold.
Glance - very little damage done, low chance of effect application.
Heavily Hits - extra damage. effects applied.
Critical Hit - lots of extra damage. effects applied.

Other Things to Note
Armor - caps out at 50% damage absorbed by armor. How much you are absorbing will vary greatly depending on your level vs the mobs level, buffs, debuffs etc etc. If you are trying to figgure out if you are capped out i recommend fighting a WHITE 1 or 2 chev conned humanoid mob and simply looking in the combat log and seeing how much you are absorbing and how much you are taking. When your armor absorbs damage, it reduces its durability.
Advantage - rolls an extra set of dice and chooses the higher roll.
Disadvantage - same thing but chooses the lower roll.
Dual weilding - You will notice that some weapons have a stat displayed as OHAC. Off Hand Ability Contribution. When you execute an ability it previously only accounted for the main weapon. Now it accounts for the main weapon and the OHAC of the offhand weapon. Positional bonus applies as normal to off hand weapons but at the ohac reduced rate. Note: each weapon positional operates independently of one another.
The crossed swords symbol - IN COMBAT. This symbol appears under your nameplate in white when you are in combat (or someone near you/in your group is in combat.) You will notice that it "drains" to grey before it goes away. As it drains to grey your health and stamina regen slowly increase (again diminished fractional values), when it is gone entirely you are at max health/stamina regen (aka you are no longer in combat). Sitting while in combat will boost this (a tiny bit) but it will also generate threat. (that exile really doesn't like it when you sit down, how rude!).
Rounding of combat log display values. Values are rounded down if the decimal value is less than 0.5 and rounded up if they are greater than or equal to 0.5. No you can not see these fractional values.
Health and Armor. Weight gain per level is on a scale and can not be calculated precisely however you can do some simple math (x-y)/a=z by looking at your class/spec tool tip to get a general idea of how much you will gain per level. This is where it says for example Defender level 1 +25 health (25 to 45), by the time you reach level 6 defender you will have gained 45 health (without gear etc boosting it) then your specialization for example Juggernaut +50 (50 to 100) by the time you reach juggernaut level 50 you will have gained 100 health. These are made up examples. The same applies to armor weight. If at level 1 defender it says +25 armor weight (25 to 45) it scales from 25-45 between levels 1 and 6 (and then from levels 6 to 50) (45-25)/6=3.333 per level, and it rounds down so you would gain roughly 3 armor weight per level. THAT BEING SAID there will be some levels where you do not seem to gain anything and then you will gain extra on another level. Don't ask me why its like this, it just is. (its because of fractions).
Remember that there are lots of fractions and variables at play and almost every stat operates as a %. Calculating things in this game precisely is near impossible since we don't get to see the true values of things and does require a lot of legwork since we can not export the logs to analyze them it must be done manually, as long as you are having fun then the rest doesn't really matter much.