A nock is a notch in the rearmost end of an arrow. It helps keep the arrow correctly rotated. It also keeps the arrow from slipping sideways during the draw or after the release. It also helps maximize the arrow's energy (i.e. its range and lethality) by helping an archer place the arrow at the fastest-moving place on the bowstring.
The main purpose of a nock is to control the rotation of the arrow. Arrows bend when released. If the bend hits the bowstave, the arrow's aim will be thrown off.