The Archimedes principle: The buoyant force on an object submerged in a fluid is equal to the weight of the fluid that is displaced by that object. This also applies when an object is submerged in a gas.
The balloon and its contents have weight due to gravity. The upward buoyant force opposes the downward weight force due to gravity, of the balloon and its contents. A balloon inflated with helium has less total mass than the air it displaces; its weight (and therefore less weightaverage density) thanalso are lesser. Therefore the air it displacesbuoyant force exceeds the weight force, the net force on the balloon is upwards, and the balloon rises.
It's a simple question, so I think it's best to give a simple answer.