PID: P=Proportional I=Integral D=Derivative. Bottom line, PIDs provide a way to hold a process to a set-point, like your AC thermostat; or to control events gradually so that a change from one state to the next is not made all at once. For example, if you accelerate from 1000 to 5000 RPM and immediately let your foot off the gas so TPS% = 0, the IAC will not immediately go to idle setting, but rather will gradually go there (note that gradually here is in a period of milliseconds). Here are two good explanations:
http://www.expertune.com/tutor.aspx and
http://www.eetimes.com/document.asp?...&page_number=2

Edit: The PIDs are within the code of the bin to control events.