Define "Filler"

Hey hey hey everybody BD

So, I have a question about fillers. What are they, exactly? They're something that I notice everyone uses, but I don't really understand the "point" of them or why they're called that.

I think that if something is part of a story, it's part of it and it's there for a reason. Why else would it be there, you know? If it doesn't contribute something then it shouldn't be there. Theoretically, this is what filler chapters are. But! All the so-called filler chapters I read are contributing something or other to the story! They add to the character's personality or they add complexity/another dimension to the story or they serve as comic relief or any other of the countless things they do. So why are they called fillers? Does this mean that fillers are just things that don't pertain exclusively to the plot of the story? Are they the "boring" parts? Or is this a mass case of paranoia about whether or not a chapter will be thought of as good or exciting? What's the deal here!? I'd love to see some opinions on the subject!

@Kewalea
To me it's really just a chapter there that can be boring but also helps lead off until the author gets a good idea for something important for the story to go after later.
I also think it can be a mixture of what you see that a filler is.
Also that a filler can be whatever you what. It's what you define and create.