A "proto-universe" or "protoverse" is a fledgling universe in the making. Before a reality comes into being, it exists as a theoretical possibility in the Celestial Nursery, physically represented by a man-sized gem housed among an infinite number of similar gems in endless recessed rows. Note that this physical representation may be nothing but an illusion conjured by Todd, the only known mortal to have ever visited the Nursery. It is quite possible that the concepts of how the Nursery functions far exceed comprehension in the mortal mind and Todd came up with the closest possible physical analog that his brain could conjure.
The same unique parameters that make a universe separate from the infinite other possibilities define the protoverse, but for whatever reason it has not yet formed a full, living existence and is thus separate from the rest of the cosmic web. Some protoverses will eventually have a "big bang" and become full universes, but according to Mischief, most just sit there "winking in and out of existence".
It is possible for an extradimensional entity who has been severely weakened or damaged to use a protoverse as a source of energy to reconstitute itself if its remains can be placed within the protoverse gem before those remains dissipate into nothingness. The protoverse is destroyed in the process, along with all the possibilities it may have contained. The implications, of course, can be grave; if a universe may have theoretically produced sentient life, countless species and civilizations may be wiped out before they even come into existence. According to Mischief, however, this is the only "safe" source of extra-dimensional energy powerful enough to regenerate an entity, implying that other sources may be available at a much higher risk to the damaged entity or those around it.