In the strip, they captured Spiff after he crashed his spaceship on a distant planet, and a high-ranking Yukbarf (referred to as "Most supreme odoriferousness" by an underling) took him to a dungeon for torture and presumably interrogation. In real life it was Calvin's father who was lecturing him on building character.
The Yukbarf specimens featured were sentient, tall masses of dripping green ooze. Coincidentally, the design of the Yukbarfs is similar to one instance of the leaf pile. The name Yukbarf is seemingly a portmanteau of the exclamation "yuck" and the term "barf", which evokes the species' repulsive appearance. The title "Most supreme odoriferousness" is most likely a referance to the smell they must have given off.