Enrage generally describes a boss ability which increases its damage output. Very frequently, bosses have a hard enrage timer to limit the possible length of the encounter, others may have a soft enrage as part of their normal abilities, and a few have both. The hard enrage is also sometimes called "berserk". Without a hard enrage mechanic, killing a boss would just be a question of having enough healers which don't run OOM. It adds some urgency to the encounter, and prevents undergeared raids from killing the boss.
The standard hard enrage mechanic for many bosses in TBC is:
- 10 minutes after the pull
- damage done increases by 500% (= six times the standard damage)
- cast and attack speed increase by 150% (= 2.5 times faster than normal)
which means that after 10 minutes, the damage output by the boss is effectively multiplied by 15.
Some bosses have a different timer length or some different ability, and a few bosses have no hard enrage at all. All bosses without it have some other mechanic in place which ensures a wipe after a certain time (e.g. the Spore Bats at Lady Vashj).
Soft enrages are a normal part of a bosses abilities and usually have nothing to do with killing him too slowly (e.g. Moroes always increases his damage output when he reaches 30% health).