Many MMO's use this gameplay concept which is basically a "holy trinity". You have characters who do a lot of damage, characters who mostly heal, and characters who mostly take damage. The latter is called a tank. A balance of the three are needed to be an effective group (mostly in the later stages of the game). The tank's job in an MMO is mostly to be the center of attention, the character which will take the damage of mobs to keep the other roles (healer and damage dealers) from taking too much damage (needless to say, the tank can take a lot of damage). Every class with the ability to be a tank has skills accessible to be able to be a good tank.
Honestly, though, a Jedi Guardian tank may not be the best choice for a beginning player. I have not idea how patch 1.2 will change the Guardian's experience, but so far, it has been difficult for a Guardian to be an effective tank. I think being a DPS (damage per second = damage dealer role) is a better choice for a new player, and you can do so if you choose a Guardian (a Sentinel may be a better DPS, as far as I know, the Guardian has a better chance of staying alive, even when he's specialized in a DPS tree).