Matching levels??

I've been puzzling over this subject for a while and have yet to hear an explanation. When you say match levels what exactly are you talking about and in what situations are you referring to? In my particular situation, does matching levels help when a track has been recorded via punch in and the punched regions,(clips) play back at different, noticeable levels. The mic wasn't moved and the singer,(me), did his best to keep mic to mouth distance and vocal volume the same throughout. I'm using a quality mic and I don't change the input level on my interface. If I get a good explanation here I will be grateful. With that said, Is there a function in Pro tools 9 for auto matching levels, either region to region or track to track and if so how is it done? Thanks