Aerodynamics is about the flow of air around an object. Imagine you drop 50 golf balls onto a big sheet of paper. Most will hit the paper. If you turn that paper so that the balls fall towards the edge of the paper (instead of the front of it) it is likely that none will hit it.
It's all to do with momentum. Each particle of air and dust hitting your vehicle transfers momentum to it. As each particle has very little mass, and your car has a lot of mass, the effect of each particle is very very small. However, when you have millions upon millions of particles hitting your car every second, the effect becomes very noticeable. The fewer particles that hit your car, the better. If you make the car very flat it will dramatically cut down on the frontal surface area, and therefore the number of particles of air that hit your car every second.
The other thing you need to remember is that air flows like water. Turn on a tap full blast and put a spoon underneath it so the face of the spoon is being hit by the water. Then rotate it 90 degrees so that the edge of the spoon is being hit by the water. Notice how much easier it is to hold the spoon in place? That's because the water just flows around the curve of the spoon instead of hitting it face on. If you make your car pointy and sharp at the front like the end of a bullet, it passes through the air much more easily.