Filler content refers to non-functional visual objects on a level:
trees, rocks, etc. Typically the only purpose of such content is to serve as obstacles or eyecandy to complement the terrain.

Attributes of filler content:
1.Static or repeating animation.
2.Cannot be destroyed
3.Cannot be interacted with.
4.Cannot be moved into/onto.(is an obstacle)

Contrast with functional content.
1.Dynamic, has states. Can respawn.
2.Can be harvested/destroyed.
3.Can be interacted with.
4.Can be moved on/climbed/etc.