"Filler" Song

What does it mean? For a song to be a "filler" on an album? I understand the concept of a filler episode/chapter on a series, cartoon or book, an episode/chapter that seems to not further the plot in any way, but how does that apply to a song in relation to an album?

For example, i've seen quite a bunch of people say that Highway Unicorn is a filler on Born This Way, but for what i thought of what would be the equivalent of a "plot" to the album, that being the imagery and overall message, i'd say it does a good job furthering that (though i may be biased as it is on my BTW top 3) since, imagery-wise, it uses the mix of urban aesthetic (highway, road, hot rods) with more fantastical and whimsical elements (unicorn, pony, rainbow syrup) plus spiritual (saint) you see all over the rest of the album, and message-wise, it's a motivational song with lots of mentions to love just like a lot of others on Born This Way, plus unicorns being associated with Femininity and the LGBTQ+ community which are talked about and celebrated on the album.

So what it means to call it, or any other song, a filler? Does it just mean that the song is forgettable? Or that it's not up to the standard with the rest of the album? Or something else that's completely going over my head?