The proto-core was a small artificial protostar, and an essential part of the protostar drive on Protostar-class starships. It featured an exotic matter dilithium matrix that was essential to the ship's ability to engage an enhanced form of warp known as proto-warp.
In 2383, the proto-core was temporarily removed from the USS Protostar by its crew and hidden inside Murf, who was in turn disguised in a reproduction of Zero's containment suit. This rendered the ship's proto-drive inoperable and incapable of warp or proto-warp. (PRO: "A Moral Star, Part 1")
After history was altered, Chakotay and Adreek-Hu ejected both the proto-core and the ship's antimatter. After getting the impulse engines and warp drive working again, the crew was forced to create a new proto-core using bosonite. (PRO: "Last Flight of the Protostar, Part I", "Last Flight of the Protostar, Part II", "A Tribble Called Quest")