In MMO games, players typically fight against creatures in the game, to be rewarded with currency or items from the defeated creature. A player is said to pull the creature when they begin attacking. They are in essence "pulling" the creature to them, to attack the creature at a closer distance. Pulling is often done to help make sure the creatures focus their attacks on one player, often a tank, who can take damage and healers can focus their healing on while everyone else in the group assists in killing the pulled creatures.
Also see: Game definitions