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

Most commonly in anime, fillers are used as a random act or episode but they are considered non-canon if the anime is based on a light novel, manga, or other source (like a game or visual novel).

"Filler" refers to story in an anime that was not in the manga or other source material. 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 popular shonen 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 generally 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.

Controversy
There is no agreed upon definition of "filler" among fans or the creator community, thus many arguments break out over what is filler and what is not, even if it is part of the source material. For long running series, some parts may appear to be filler, but then turn out to be more relevant as the story develops, so even the most obvious filler can't definitively be labeled as such until a manga or series is complete.

Also, if the anime is an original work, a comparison with source material can't be done, but fans will still insist on labeling episodes or parts as filler based on their own unexpressed definition or based on value judgements unique to them.

Some filler content may actually be authored or approved by the author of the original source material, but fans will still label it filler.