The term 'filler' is used in anime episodes to highlight events or occurrences that were not adapted from the source material, be it manga, light novels, or web novels; they often feature less significant adventures or situations that won't impact the development of the main plot.
In anime production, filler is created by the animation studio in charge, and in some cases, it is done in collaboration with the manga or light novel author. This is done to ensure that the story remains consistent and does not contradict the main plot.
Filler can become canon if it is accepted and acknowledged as an official part of the story by the creator of the manga or novel. However, this is relatively rare, as most fillers are seen as additional stories that do not impact the development of the main plot and are not taken into account in future events.
Why are filler episodes created?
Filler episodes are primarily created to allow the manga or novel author more time to produce additional material for the anime adaptation. This helps avoid halting the broadcast and keeps the anime continuous. Some examples include Naruto Shippuden, Bleach, or One Piece.
If the production team doesn't have enough manga material to fill a complete season, they may create filler episodes to fill the gap.
Is all filler bad?
Not necessarily, some filler episodes can be very entertaining, adding more depth to the characters and the world in which the story unfolds. However, it is also true that many of these can be considered irrelevant or unnecessary in relation to the main plot.
Some fillers can be seen as an opportunity to explore secondary stories or less important events, while others may be viewed as a way to fill time without adding any value to the series.