Pitting is when you go to the range with your hair perfectly brushed, make-up on your face & clothes that look like you just walked off a movie set that stars Brad Pitt. I try not to Pitt if I can help it. In Georgia, Pitting can get you beaten up if you aren't paying attention & lower your guard (and your gun).
Hope that helps you.
Seriously, pitting is when rust is allowed to stand on the metal long enough to eat into the metal. I have a 22LR that was left in my parents closet for years with no oil on it. The surface rust just kept growing and eating the metal until I found it a few years ago. If I had found it sooner I could have cleaned it off with steel wool and some oil. Now I have a 22 that has a pitted barrel. Keep oil on the metal and you will never get pitting.