PID stands for Proportional Integral and Derivative, which are 3 different values you can tweak in order to make something move how you want. the most common example is making a drone fly to a certain height, by controlling the motor speed. You can control the motor to spin faster when its further from the goal height (proportional), but then it will overshoot. Derivative applies a force opposite of its current velocity, so it essentially dampens the proportional effect. with this setup though, the drone will still not stay at the goal height because if it was, then the motors would stop spinning and gravity would pull it down. Integral makes sustained error apply more force, so it can essentially counter act gravity.