An episode that you could remove without noticing. There is no impact to the overall plot. No characters are changed in any way. It's never referenced again.

Many shows like sitcoms and procedurals are designed this way. Filler is not inherently good or bad. Often when it's an adaptation of something, I find the filler to be bad. But in some shows, filler is the best stuff. I couldn't care less about the overarching X-Files plot, but the episodic episodes ruled.