While riding the train, some girls get into trouble.

What does a filler mean in anime?

What you mean is a “filler”. Filler in an anime has a wide definition. Fillers are episodes showing content which was not written by the original author or in other words is not ‘canon’. Filler episodes are mostly used in Anime to give time to the Author/script writer/Mangaka to produce more content.
when the Anime reaches on par with the manga, The anime team has two options, That being either to go on a hiatus until more content is produced, or Air filler episodes with random story, not related to the main plot so that it cannot effect the original story.) But sometimes the filler episodes are produced by the studios to commemorate special occasions and festivals.
For example Christmas special episodes, New year specials as well as Golden week specials. And rarely it is used to give the audience a rest from the main story.