Don't let that image intimidate you. It's not that bad! I was surfing on the Parallax Propeller forums and I noticed a post on PID controllers for UAVs that might be useful to some of you:
PID Stuff for you UAV Nuts out there!
PID controllers are commonly used in UAVs to keep the airplane on a specified path. Sniper King discusses each of the three parts of a PID controller in easy to understand language. He also gives advice for flight testing your software and tuning your gains.
I haven't looked at the PID code in the Parallax OBEX, but I believe there is an alternate PID form that uses only one gain. For a more thorough explanation on PID controllers, you may want to check out the wikipedia article