When you press the button, you warm the air in the balloon and it flies into the air!
When the air heats up, the air particles move faster. Then they need more space: the air expands. Some particles find a way out through the hole in the balloon. Since there are now fewer air particles in the balloon, the balloon becomes lighter and rises.
And when the air cools down? Then the air particles creep closer together, in the balloon as well. That’s why the balloon comes down again.
Expansion of ice
Air expands when it gets hot, but strangely enough, water expands when it freezes. That’s why you should never put a full bottle of water with a lid on in the freezer. If the water expands, it can no longer escape and the bottle is likely to break.