I am deeply frustrated at the increasing usage of the concept of "filler" in writing, partly because how people are using it is completely separated from what it usually means, and partly because few people explain what they actually mean when a story lacks "filler."
So, in short, when this comes up, what people usually are talking about is:
A lack of subplots. Subplots are essential background to developing your story beyond the main action points. Subplots include romantic storylines, internal journeys (learning to deal with a trauma, for example), plots that are critical for secondary characters, etc.
No room to breathe (aka pacing). Keeping up the tension is a common piece of advice, but if every single scene is filled with tension, you're going to exhaust the reader (and yourself). Quieter moments allow characters to reflect, realize things (like feelings for each other), and give you space to establish worldbuilding and mood.
Rushed or missing character development. Giving your characters space to learn things about themselves, confess or realize feelings, and moments to make decisions and change are essential to a story. Defeating the big bad is important, but usually the inner strength to do so comes from the character changing over the course of the story.
I am losing the battle against using the word filler when you mean a story lacks other things. Other than shaking my fist at the youths, I suggest that if you get feedback about "filler," see if you can get specifics. Is it an issue with pacing? Character developments? Not enough description or worldbuilding? If a romance seemed rushed, at what points does the reader think there should be more interaction? Etc.
A note for fanfic writers: Your goals are often different from someone writing to publish, but hammering down what filler means can still help you as well. Narrowing down the point of your story ("in this drabble, these characters flirt without using words" or "sexy coffeeshop AU") and making sure to ask your beta readers if what you're writing fulfills that goal can help you figure out if your story is making the impact you want.