Peeping is the sound they make -- the other is the act of puncturing the outer shell of the egg with their beak and "egg tooth" (the little horny knob on the end of the beak when hatched, it pretty much functions as a can opener for the shell, and falls off in a few days) to make an air hole to breath.
So, you should have both going on simultaneously if you're going to have a healthy hatch.
Internal pipping is when they break through the membrane into the air cell, they usually start vocalizing(peeping) at this point. External pipping is when they break through the shell.