You will need:
All objects try to stay in one place unless a force makes them travel somewhere. This is why when the car goes forward you 'feel' as if you're being pushed backward, but really your body is just trying to stay where it was before the car moved. This is called inertia.
Inertia also affects the air inside the car. When the car goes forward, the air inside also tries to stay resting in the spot where the car was parked. As the car accelerates forward, the density of the air increases towards the back of the car. This builds up pressure in the back of the car.
You would expect the helium balloon to travel to the back of the car as well. The problem is that the helium inside the balloon is less dense than the air surrounding the balloon. The balloon tries to 'float' above the dense air in the back of the car, causing it to go forward.
Check out more on flotation and how the Archimedes Principle affects how boats work.