A filler episode is basically a pointless episode, although this is slightly open to interpretation. In general, if something important happens that may affect later episodes, then it is not a filler episode. Examples of non-filler episodes can include:
- Ash has a Gym Battle
- Ash competes in a League
- Ash fights a Rival
- A main character (Ash or a friend or Team Rocket) catches/evolves a Pokemon
- A main character releases a Pokemon
- May/Dawn/Serena fights in a contest (this can be debatable as to whether or not it is filler)
- An important event with an evil team happens (doesn't include Jessie/James/Meowth, unless they are being directly ordered by Giovanni, as happened with the Meloetta stuff)
Other things might also come into play, as well, for example how important is it when one of a main character's Pokemon learns a new move? Also contests are a bit of a gray area as to whether they are filler or not. So things like this largely come down to personal preference.
There might be other stuff that isn't considered filler, but this is the main points. At least that I could think off of the top of my head. Basically if the episode doesn't fit one of the above examples, then it is a "filler" episode.