Can somebody tell me what a MAP sensor is for and what it does?

It measures the amount of changes in vacuum pressure in your engine and sends signals to your computer to help with fuel delivery. If it fails your engine can idle rough, unstable, and set trouble code(33 usually). On Corsicas it usually has a vacuum line running to it and these can crack and leak over the years. If your are having problems with idle would check for codes stored, clean IAC, and replace any old vacuum lines, as well as intake (rubber hose) assy. if cracked. If your MAP is bad, they are usually pretty cheap and very easy to replace.

thanks.