It's where they hop from all fours (sorta) and is usually an indication they're happy. Baby chins do it a lot, few of my adult chins do. Youtube has some videos you can watch of popcorning in action.
Some of them even give a little wiggle at the top of the jump. It is really funny to watch. They literally bounce all over the cage like popping popcorn.
Ha! I thought popcorning referred to the crazy hopping that one of my chins does when she is let out for playtime, but I wasn't sure until I saw the video. She does it about 5 or 6 times when she if first let out. It ALWAYS makes me laugh!