The diagram below has the air particles in front of the car represented as blue dots. In reality they are much smaller, too small to see, and there are much more of them, millions and millions in fact.

The diagram is representative of the particles but not to scale.

As the air particles are in front of the car, the car must push them out of the way so it can move forwards. As the car pushes each particle, each particle pushes back with a small force. As there are so many air particles, the addition of all these small forces is a significant force. We call this force air resistance.