Think about it: The whole plot of Pokemon is Ash's journey to become a Pokemon master and usually when he helps people (Which is what people consider filler) he learns something about Pokemon which helps him get closer to his goal right?

A filler episode is an episode where nothing of any importance happens. Ash learning not to pressure your pokemon into baking a cake otherwise it will run away is not important.

Ash learning that Poliwrath is water AND Fighting 5 times isn't helpful. Neither is giving random kid #642 the confidence he needs. And dressing as a girl (except the first time) isn't included in the stuff you need for becoming a pokemon master.

Or things that uasually delay Ash to his destionation, getting lost in the woods, missing a ferry and having to find alternative means of trasnsport. Also having a road or route that could be lessen the time it takes to get the destination closed down. Yeah stuff like that can be considered filler.

Look through my blog for season guides. See all the episodes I skipped? Those are completely fillers.