What does the tank do?

Answer 1

The tank's job is to get ... and hold ... aggro. Tanks tend to be either heavy armor classes with lots of health (who can take a lot of hits), or light armor classes with very high Dodge abilities (who can survive a lot of ATTACKS because most of them don't actually hit). Most classes intended for tanking have abilities to increase their perceived threat, or to otherwise somehow persuade an enemy to attack them in preference to other players.

That may not be what you meant at all, in which case consider it an object lesson in asking detailed, specific questions so we don't have to guess what you're talking about.