Filler

Sakura: So, did we even do anything this episode?

Kakashi: Sakura, welcome to the wonderful world... of filler.

Everyone: Nooooo!

—  Naruto: The Abridged Series

In anime, fillers are used as a random act or episode but they are considered non-canon.

"Filler" refers to story in an Anime that was not in the Manga. These are created because Anime production usually outpaces the Manga. Fillers do not further the story and usually are of a lower quality both visually and narratively.

Media
Filler episodes are entries in a generally continuous serial that are unrelated to the main plot, don't significantly alter the relations between the characters, and generally serve only to take up space. This could be considered Padding applied to a whole franchise.

They are extremely common in Anime, where many shows have 26 or more episodes per season. The producers have to use filler just to meet contractual demands. Filler is usually something entirely original for the anime, but not always; many manga - particularly weekly manga - employ filler just as ruthlessly due to the extreme deadlines. Sometimes entire filler Arcs are created, most often because the series Overtook the Manga. Just about every long-running manga-based anime action series will have gargantuan amounts of filler over time. This is because Japanese networks, unlike western ones, don't do reruns or season breaks. This is compounded when they go beyond the 26 episode mark. Many series air over 40 episodes per year, when they would have a hard time making even half of them related to the main plot. In most cases, the defining aspect of filler is the lack of series momentum. Filler can be safely ignored without any loss of important information. However, there is also a style of filler called the "single upgrade filler". This uses a filler episode to introduce a new power, machine, costume, minor character, etc. without having to work it into the greater narrative. In these cases, the episode can be ignored outside of "something got an upgrade".

The term "filler" is also used by fandom to refer to  anything  that isn't in the source material. This stems from the practice mentioned above of adaptations that are threatening to catch up to the source using original story arcs, episodes, and general content to pad things out. That is not this trope, see Overtook the Manga or Adaptation Expansion, but such cases are often related to it. Well-known anime's such as Naruto, One Piece, and Bleach are known for their filler episodes.