If you remove the entire (episode/arc) and the story isn't affected in any meaningful way it is filler.

For an easy situation: Imagine you are making a series about Robinhood. It starts as him as a child training to be an archer until he eventually fights the sheriff of Nottingham. Throughout the series his bounty goes up as he becomes more renowned. If you add 5 episodes that focus on him teaching a child to fish and at the end of the arc he realizes the kid sucks at it so he steals the prize money to give the kid anyway then it is filler. It doesn't add to the plot when it gets back to the main story