PID algorithm is a Control System based dynamic control algorithm. In simpler terms it can be described as a dynamic error calculation and correction algorithm/

Majorly used in wheeled robotic locomotion to follow and maintain locomotion along a straight guideline usually drawn on the floor.

The code here is a very basic implementation of the PID algorithm. A very good algorithm for robotics enthusiasts struggling with robotic locomotion.

Do read the detailed documentation on the concepts and tuning formulas on: https://en.wikipedia.org/wiki/PID_controller