When you let it go, the air at high pressure inside the balloon escapes at high speed and the balloon is pushed in the opposite direction to the escaping air. The action of the ballon on the air produces a reaction of the air on the balloon.

Cars operate on the same 'action – reaction' principle. A car is not though, made to move because the exhaust gases are ejected at high speed. Instead, as long as the tyre grips the road and does not spin, the tyre exerts a backwards force on the road, resulting by Newton's Third Law, in an equal forwards force being exerted by the road on the tyre. This happens at every wheel connected to the driving force, so the car moves forward.