36SG-555 DC motor with magnetic encoder ; code issue pls Help

Hi, I did some new Modifications on the motor with the encoder , So I add the pid calculation and the attachInterrupt function what i didn't understood it very will is the PID calculater , what I know is that the PID calculation (Proportional-Integral-Derivative ) is a mathematical formula used to control an output to do something , so Proportional calculate the error , and Integral-Derivative of the error WHAT IS A PID CALCULATION ? HOW WE CHOOSE KP KI AND KD ? WHAT DOES MEAN A Proportional ? WHAT DOES MEAN A Integral? WHAT DOES MEAN A Derivative ?

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