A plot continuum (plural plot continua) is a distinct reality consisting of all the events taking place in a given fictional universe over time. Each plot continuum is self-contained, and may or may not bear any relation to other plot continua.
Each continuum is defined by its canon, an original work or body of works set in its reality. The canon lays out the rules of the universe, such as how physics works, whether magic is possible, whether there is absolute good and evil, how people live, etc. The canon also tells the story of some of these people. The story makes a universe into a continuum; without a story, a universe would just be a place.
Non-canon intrusions on a universe, such as bad fanfiction, can destabilize a continuum by breaking its rules and twisting its story. When this happens, the Protectors of the Plot Continuum step in to remove the negative influences, allowing the canon to snap back into place.