Gravity makes a toy car go down a ramp. It's the same force that keeps objects stuck to the ground and makes dropped objects fall.
Gravity is a natural attractive force between two bodies. In this case, the bodies are the car and the Earth. Surprising to many, gravity is the weakest natural force known, It is only evident in our daily lives only because the Earth is so massive: about 5,000,000,000,000,000,000,000,000 times as much as a rather large 1-kg toy car. In space, without the Earth's gravity, two toy cars would attract each other, but the force between them would be so weak it would take many years before they touched.