Tank characters deliberately attract enemy attention and attacks (potentially by using game mechanics that force them to be targeted) to act as a decoy for teammates.
Since this requires them to endure concentrated enemy attacks, they typically rely on a high health pool or support by friendly healers to survive while sacrificing their own damage output.
Since they keep other members of a team alive, tanks often take on an unofficial leadership role:The tank acts as the de facto leader of the group by pulling and holding monsters' attention.
I'm also expected to stay on top of all the current meta strategies for beating a dungeon.
[2]The term was used as early as 1992 on Usenet to describe the warrior class on BatMUD.