Have you tried the FIRST wiki? When I learned about PIDs last year, I found that very helpful. try this
link.
Essentially, what you need to know is that PID controllers make sure you move a motor (or robot, or something else) to the exact right spot using a sensor. It makes sure there is no over--or under--shoot by giving you real-time feedback. I don't know if you are using the WPIlib for C++ or not, but if you are there is a really good class that implements a PID controller for you.