Filler is anime material that does not adapt the source work: original episodes, scenes, or whole arcs created so a weekly anime does not overtake the manga or novel it is adapting. The term describes origin, not quality.
Filler exists because of arithmetic. A weekly manga publishes about four chapters a month; a weekly anime episode can adapt two or three of them. An anime that never pauses will catch its source and run out of story, so productions insert original material to hold the schedule while the manga builds a lead. Entire filler arcs, sometimes dozens of episodes, are the long-running shonen solution to that equation.
The fandom's relationship with filler is complicated. The label is factual, not a quality verdict: some filler is disposable, and some is beloved, written by strong staff or expanded from the author's own notes. Fans maintain episode-by-episode filler lists for the big franchises precisely so viewers can choose their own path.
On this site, filler is never quizzed as plot. It appears only in the Canon or Filler format, where identifying it is the game, because treating invented episodes as canon facts is how quizzes quietly become wrong. A series' canon sheet lists its filler ranges so the boundary is never a guess.
In use"A beach episode invented by the anime studio between two manga arcs is filler, whatever its charms."