A filler episode in television is an episode in which nothing that may affect the progress of long story arcs or the development of the main characters with no returning side characters of other important ones appear in the episode.
Filler episodes are any episodes in a season of television which does not contribute to the overall plot or thesis of the television show.
So in a television show without any source material, if you're able to skip watching the episode and not miss any of the plots relevant info then it's a filler episode.
For example such as with anime.
The filler episode would be an original episode or story that is created for an anime which is not based on the original manga.
The filler episodes are often created when an anime series catches up to the manga and needs to wait for more manga to be published before they continue the story.
Filler episodes for television shows are common in long running anime series shows like Naruto, Bleach and One Piece because anime is generally created faster than manga.