Tank is gamer-slang for a player that is geared with emphasis on Stamina, Physical Defense, and Aggro Management skills so that he is very good at holding the monster's Hate while the other players pound it into chitlins.
The classic separation of duties in a group is:
Generally, a group only needs one Tank and Healer, but may have more than one of each type. Ideally, in perfectly-functioning group, the Tank should never lose Aggro, and the Healer should never need to target anyone except the Tank. Of course, we do not live in a perfect world, and "... specialization is for insects." — Heinlein