A ratchet is a gadget with mechanical teeth on a wheel that moves in only one direction. Therefore, ratcheting is to move something slowly in one direction.
A ratchet is a device that allows you to move or crank something in a specific direction: a ratchet only moves one way. You can tell a ratchet by the wheel of mechanical teeth. Ratchets are part of larger devices, especially in factories. Also, if you ratchet something up or down, you're gradually moving it in that direction. You can ratchet something physically, or a great speaker could ratchet up the energy in a crowd.