A run cycle in After Effects is a looping animation of a character running. It typically involves creating keyframes for the movement of the legs, arms, and torso, and then animating them in a seamless loop. With the help of tools such as the graph editor and the loop expression, creating a run cycle in After Effects can be a straightforward process. This animation is often used in video games, commercials, and other forms of media to add movement and energy to a character or scene.