I am new in this site, and like you can see I want to do a line follower, not a clumsy and slow line follower, but a fast and "fancy". I investigated about the PID method, but can someone explain me in simple words what is it. Thanks for your answers.
Did you check wikipedia, for example?
I doubt it is possible to describe a PID controller in simpler words than those used by the article.
The pseudocode is quite simple and should be also very good for understanding the concept, when applied to a digital system.
See also the limitations of PID controllers.