The small bits of food left over on the plate, or the bottom of the frying pan, inside the toaster, etc., which normal people don't eat without a little peer pressure.
1. "Here buddy, eat these leavins so we can make another batch."
2. "You have to eat all the leavins if you want to get the free t-shirt."
3. I didn't even know my toaster had a cleanout flap, so when I finally discovered it there were enough leavins in there to make a whole meal. But I wisely chose to leave them leavins in there, so the years of flavors come out in each and every thing we toast.