Backlash

Backlash is the tiny gap between meshing gear teeth. When a joint reverses, the motor turns a little before the teeth touch again, so the arm briefly does nothing.

You can feel the same thing in a loose bicycle chain or an old steering wheel.

Precise robots try to keep backlash close to zero. As gears wear, backlash grows, and a robot can start to click on reversal or miss its marks.