"Filler episodes" refer to episodes in a TV show or anime series that do not significantly advance the main plot or contribute to character development. They are often used to buy time or fill gaps between major story arcs. Full definition
Each quest in this MMO is like an episode of the show, and each mission series is an arc, complete with the occasional filler episode. (pcgamesn.com)
Every episode, while a few a few seemed like filler episodes, has (metacritic.com)
Fairy Tail's anime has been stockpiling omakes and filler episodes in the past month, killing time before moving on to the next big arc. (goodereader.com)