Total Item Level (TIL or IL)

Total Item Level (TIL), which is the sum of the item level of everything you equip (from equipment, to mount and companion powers... almost everything gives you item level) is the most important thing in the game. It directly impacts your damage and your Hit Points (HP).

Damage = TIL / 10 (plus a 20% bonus for Damage Dealers, and a 10% bonus for Healers)

The Damage stat is a multiplier that is applied to both the damage you deal and healing you provide.

Hit Points (HP) = ( TIL x 10 (plus a 20% bonus for Tanks, and a 10% bonus for Healers) + HP given by items ) x ( 1 + Constitution / 200 ) + 1

You start the game with 5000 TIL, so you start with 50k HP and you deal 500 Damage, from TIL, plus your bonuses from race, ability scores, and role. Your role depends on the class you play, many have a second role you can play starting at level 20.

The higher your TIL, the less an increase will be effective, and the more you'll need to benefit from an increase in damage or TIL.