It is where you re-record a small bit of a track leaving the surrounding material untouched. I generally use this technique to edit our unwanted notes in an other wise good take. It is another reason to keep good studio logs as you want to make sure that the input signal strength, effect levels and eq are all set the same for your punch piece as they were for the original or you won’t get a seamless edit.

If you can use a scrub view the best way to pun in is to pick a point where the sound wave has a "zero" value and this will ensure you don’t get any "pops" !