"A Jedi uses the Force for knowledge and defense, never for attack." - Master Yoda
The light side of the Force is a path the player can take in Jedi Knight: Dark Forces 2 based on their actions in early parts of the game. If the player protects civilians and kills the enemies, they follow the light side path. This also gives them the ability to use light side Force powers, but can't use Dark side Force powers.
The force powers that are exclusively available to the light side are Force Healing, Persuasion, Blinding, Absorb and Protection.