Respec is slang for respecialisation and refers to games where you have a skill, 'talent', 'stat', 'feat' or similar system that you develop as you level up. Respeccing is in that case a matter of resetting all the skill points, feats or the like, and start from scratch, without having to reroll your character.
The word is mostly used in RPG-type games, such as Diablo 3 or World of WarCraft.