What is the RAM disk?

Sorry for the basic question, but:

I'm following Apple's steps to get my iBook G4 to eject a stuck CD. In the instructions to reset the PRAM, it says:

Warning: Resetting the Power Manager on any PowerBook or iBook will permanently remove a RAM disk, if present, and all of its contents.

That doesn't mean it will erase my hard drive, does it? Not sure what a RAM disk is.

thanks,
david