ON/OFF control mode is simplest form of automatic control mode, it's like a switch, either ON or either off. the rules is very simple, in a reverse control situation, when prorcess value is lower then the desirved, the output is fully on, when process value higher than the setting value, fully off. an analogy we often use is like driving the car from starting point to finishing line.

In the situation of an ON/OFF control, you apply the full thrust and drive the car to the finishing line, and after the car cross the finishing line, you release the thrust and turn the engine off. the rule is very simple, the downside of this type of control mode is there will be significant overrun at the finishing line, it's impossible to stop on the finishing without overrun, in contrast, the PID control mode, the driver will observe carefully and calculate the distance and speed in relation to the finishing line, and gradually apply less thrust and maybe apply a little brake too to make sure the car stop right on the finishing line. it's a constant adjusting and fine tuning process.

Another analog to use is Filling a Glass of Water

• ON/OFF Control: You look at the glass. If it's not full, you turn the tap on full blast. The moment it's full, you slam it off. You will always overshoot and get wet.

• PID Control: You look at the glass and adjust the tap flow continuously. You start with a strong flow, then reduce it as the glass nears full, finally trickling the last few drops to hit the exact level without spilling.

  To summe this up, ON/OFF control is a simple, low-cost, "good enough" solution for non-critical, slow-responding systems where cycling is acceptable.
  PID control is an advanced, precise solution for critical, fast, or efficient systems where stability, accuracy, and minimal oscillation are required. It's the workhorse of modern industrial automation.