Light Screen reduces damage from Special attacks by 50%, for 5 turns. Its effects apply to all Pokemon on the user's side of the field.
If the user is holding Light Clay when Light Screen is used, its effects last for 8 turns.
The move Brick Break, when used against an Pokemon under Light Screen's effect, will remove the screen before dealing damage. The ability Infiltrator bypasses the effects of Light Screen.
Light Screen is the Special counterpart to Reflect.
Basically if your Special Defense is 100 and you use reflect your Special Defense will turn into 150 for 5 turns.