PID control maintains accurate and stable levels in processes (e.g. controlling temperature) by combining Proportional control with Integral and Derivative action.
Proportional control avoids the oscillation characteristic of on-off control by continuously adjusting the correcting variable output(s) to keep the process stable.
Integral action eliminates control deviation errors over time, and derivative action counters rapid process movements. To achieve optimal control, the PID three terms must be tuned to match the conditions in the application.