What is ratchet and pawl mechanisn?

A pawl and ratchet is used for example in a clock spring winding barrel. A toothed wheel with barbed triangular teeth are cut into the circumference of a metal disc attached to the axle of the spring barrel. This is the ratchet. The pawl is a pivoting teardrop shaped piece of metal mounted on the clock plate adjacent to ratchet. The pawl drops into the gap between the teeth of the ratchet as the clock key turns the spring barrel. The pawl clicks over the teeth until the key turning pressure is removed. The pawl then drops into the last notch in the toothed wheel and stops the spring unwinding.