A ratchet is a special device that allows motion in one direction, but prevents it in another.
Here is one example of a ratchet. A short axle is meshed with a 16 tooth gear. A white Lego belt holds the ratchet against the gear. The ratchet allows the gear, and the beam attached to the gear, to rotate in one direction, but prevents it from rotating in the opposite direction. Ratchets are useful for holding things up against the force of gravity.
See also: Worm gear