Popcorn is a colloquial term for XP-valuable monsters of a low level that are easily killed.
Popcorn is usually the result of careful play that takes advantage of the bonus experience gained from killing high-level monsters exclusively. The surplus lower-level monsters can usually be killed in a single strike, incurring no loss of health. With a large popcorn reserve, players can harvest enough popcorn to level up once or even twice in quick succession.
This is a common strategy for fighting bosses or high level enemies, often called a 'mid-level ding', where a player depletes their resources before killing enough popcorn to gain a level and receive full health and mana.
Popcorn can also synergize with many gods who grant large amounts of piety for killing enemies. Popcorn is a reserve of piety as well as XP and even health for characters with life steal
Using WONAFYT during early exploration create what is known as a 'popcorn bowl': a central reserve of slowed monsters that are always accessible and easily harvested. This is one of the primary uses for WONAFYT and can be a very powerful resource during the late game.