On a bicycle, if you stop pedaling, you don't immediately stop. You keep moving with the same velocity you had just before you stopped pedaling, and then you gradually slow down. Slowing down is due to forces acting on your bicycle--wind resistance, friction in the wheels, etc. But if there were no force acting on your bicycle to slow you down, you would continue to coast at a constant velocity once you stop pedaling. But since there are forces acting to slow you down such as wind resistance, maintaining a constant speed requires you to pedal just enough to cancel the resistance force--when one force cancels the other, you are left with zero net force and thus remain at a constant velocity.