The freewheel is the part of the drivetrain that allows the bike to "coast" without the pedals spinning. This is normally achieved by the implementation of a ratchet based system, that allows drive to the rear wheel with a clock-wise crank rotation.

Freewheels are either mounted on the cranks (front freewheel) or ont he rear hub ( rear freewheel). Freewheels rey on a threaded mounting system, and so can be very difficult to remove due to them tightening during pedalling.