Mechanical watches that are automatic or self-winding rely on the natural motion of the wearer to keep the watch wound. The watch mechanism has an oscillating weight that turns when the person wearing the watch moves their wrist. The movement of the oscillating weight keeps the mainspring wound. If the watch is not worn for a period, you may turn and/or wind the crown a dozen or so times to start the watch.
The mechanical watch movement has a mainspring that must be wound. As the mainspring unwinds, it provides force to a series of gears that power a balance wheel which oscillates back and forth at a constant rate. This oscillation causes the ticking sound that is heard in a mechanical watch.