A 'filter episode' is an episode in a series that doesn't quite add anything to the storyline or have anything to do with the main plot. It is merely an episode that appears to be something to lengthen the series, or an episode that is used because
the producers have run out of ideas and
do not want the story to conclude just yet.
Example 1:
Jeff: "Dude, yesterdays episode was crap! It didn't even mention what
happened last week!"
Ken: "Must've been a filter episode"
Example 2:
Stacey: "Ugh, another filter episode! When are they going to continue
the story?!"