Rubickian Prison Boxes is a type of enchanted object which exists in the 2017 Continuum.
A Rubickian Prison Box is a cube which can be used to imprison powerful demons. It resembles an ordinary Rubik's Cube toy, and, if solved like a Rubik's Cube, will release the demon contained within it. Rubickian Prison Cubes presumably originated on the Rubickian Puzzle Planet in the Yonder Galaxy.
Behind the scenes
Rubickian Prison Boxes were first mentioned by Frank Angones on his Suspenders of Disbelief blog. According to Angones, if Webby were to find a Rubickian Prison Cube and realize what it was, she would solve it anyway. They are presumably meant to be a reference to the Rubickian Puzzle Planet, mentioned in the Wander Over Yonder episode The Rider.