Stats - statistics - are the numbers used by the game to determine the effects of an attack you make on a mob; or a mob makes on you. You'll find stats on all of the equipment you use in-game, and as you have leveled you probably have noticed that the size of those numbers has increased.
But sometimes, you have to choose between pieces of gear or weapons that have similar AMOUNTS of stats, just different ones. Perhaps one piece has might, vitality, & evade; while the other has might, fate, & crit.
When choosing gear -- particularly if you're choosing what to use for an endgame character going into level-cap instances -- you need to know what the stats on your gear are doing for you. If you just pick pieces with high stats, you may find that while your GEAR is good, your BUILD (the composite way your stats work together to affect your role, whether tanking, healing, or doing DPS) doesn't work the way you wanted it to.
There are some very good, very detailed pages already in existence.
- Character Stats will give you lots of great information about every single stat, and how it affects specific classes and and the roles they play
- Stat Derivations shows the amount of secondary stats you receive from your primary stats (primary stats are might, agility, will, fate, & vitality; secondary stats are the rest)
- Stat curves will show you how much of a stat you need to cap -- and how steep or shallow the curve is. I use those a lot to decide between two different essences, see which is going to make a greater difference at the amounts I'm at
But it seems all of those lack a very basic description of what the stats in your character panel do, so I'll do a bit of extra work anyway in the hopes it will help someone out.
Your Character Panel (default keybinding C) is the way to keep track of your toon's cosmetic armour, equipment armour, jewellery, weapons and stats. When you pull it up, on the right-hand side is a list of the total stats your toon currently has. Make sure the checkbox at the top (Show All Stats) is checked or you're going to be wondering where some of these come from!
Morale: your toon's hit points; any damage you take decreases your morale; if you get to 0 you are "demoralized", or dead, and need to revive or retreat.
Power: your ability to perform your skills. Each skills uses power and if you run out or are drained you cannot do skills until it regenerates; Beornings are the exception -- their skills build & use wrath.
Armour: the overall defensive capabilities of your gear; physical mitigation is increased by 100% of your armour rating; tactical mitigations increase by 20% of your armour rating.
Might: varies by class, see the Character Stats link above
Agility: varies by class, see the Character Stats link above
Vitality: varies by class, see the Character Stats link above
Will: varies by class, see the Character Stats link above
Fate: varies by class, see the Character Stats link above
Critical Rating: a critical attack deals at least 150% of the tooltip damage for that skill; a devastate starts at 200%. The chance of getting a critical or devastating is determined by your critical rating. The amount of damage dealt (magnitude) also increases with crit rating. Check your tooltip to see where yours is currently at.
Finesse: Finesse reduces your opponent's Block/Parry/Evade (if you use physical mastery) or Resistance (if you use tactical mastery); the more finesse you have, the less often the mob will B/P/E/Res. your skills
Physical Mastery: decides the strength of your physical attacks
Tactical Mastery: decides the strength of your tactical attacks and heals
Light of Earendil: Neutralizes shadow; see the Light of Earendil guide; Light is only an issue anymore in Mordor landscape and certain Mordor instances
Resistance: your chance of resisting -- or totally negating -- incoming tactical damage
Critical Defence: reduces the amount a mob's critical attack hits for;it does NOT decrease the CHANCE of a critical attack, only decreases how much a critical attack is amplified by
Incoming Healing: amplifies the amount of healing sent your way -- by yourself or others
Block: the ability to use a shield or 2-handed weapon (some classes, some trait lines) to stop a physical attack, negating the damage; caps at 18%, although partial block can continue to increase; you cannot block attacks from behind you
Parry: the ability to use your weapon to push aside a physical attack, negating the damage; caps at 18%, but partials can continue increasing; cannot parry attacks from behind you
Evade: the ability to "move aside" (character doesn't literally move) and negate a physical attack; caps at 18% but partials can continue increasing; you CAN evade attacks from behind
Physical Mitigation: if you don't block, parry, or evade a physical attack (or if you get a partial BPE) the damage is decreased by the % mitigation you have
Tactical Mitigation: if you don't resist a tactical attack, the incoming damage is decreased by the % mitigation you have
- Light armour classes cap at 40%
- Medium at 50%
- Heavy at 60%
- Class traits that add a % of mitigation, add it directly to your tooltip percentage, so you can actually over-cap that way
- T2 instance mobs ignore about 7,000 mitigations, so to cap in those instances you need to add 7,000 to whatever you cap at. The Character Stats link above has a table showing the exact amounts necessary.
I hope this has given you a bit of insight into how the stats work together to make your toon stronger. If you really like seeing how the numbers work together, Dadi has done an excellent job in his Guide to Damage, Mitigation, Avoidance, & Calculation. I never could have figured this out myself but as far as I can tell it's all accurate, and has been helpful to me in understanding how the game is working.