A patch is a file which you add to a cd image on your hardrive
to alter the game.
Patches are used to allow the game to be played from another region [PAL/NTSC Selector], other patches are used to add cheats to the game [Trainer] and some patches are used to remove the copy protection from certain games [Protection Fix].
If you need to obtain any patches you should check out
www.megagames.com. Sorry I don't have a link for a tutorial for you, I'm sure someone else does.