The fists are a melee weapon in Jedi Knight: Dark Forces 2 andMysteries of the Sith. These are the final resort after the Bryar pistol. These would only be used in a "life-or-death" situation where the player has absolutely no ammo left, as they enflict very little damage to enemies. However, this would only be the case up until you get the lightsaber. The fists are recieved at the beginning of the game along with the Bryar pistol.
Fists do damage directly to the health of the target, bypassing shields (useful info in a guns-only multiplayer match). Something to watch out for in single player as well, is enemies who do physical damage (Kell dragons, Gamorrean guards, types of Grans, etc.) also ignore the shields of the player.
In Dark Forces 2, the fists alternate between punches with the left and right hand/arm. In Mysteries of the Sith, primary fire consists of weak rapid punches with the left hand, and secondary fire is a slow (and more powerful) punch with the right hand.