PID is a closed loop stability algorithm that "smooths" automated control of a process. Ie, it prevents under-overshoot of say a temperature controller, or speed control of a motor controller. It attempts to predict the dynamics of the controlled device to provide a smooth outcome with varing load conditions. It can be difficult to tune manually, most industrial controllers provide an "autotune" function to set up. Manually, suck-it-and-see value input is the only way, with the process in its typical operating enviroment.