What is Filler?

1. Anime is typically supported by Manga, a kind of Japanese comic favorable by individuals of all ages.
2. "Filler" refers to a story in an Anime that was not in the Manga. These are created as a result of Anime production sometimes outpaces the Manga.
   
3. Fillers don't continue the story and are typically both visually and narratively of lower quality.
   

What is a canon?

A canon is the core of something. The thing that's definitely true to the story.
Non-canon are e.g. what-if-stories, fan fictions, spin-offs that are distinct (in their own universe) and so on. Stuff that is related to the product, but not actually part of it.
Now, with Filler, there is some muddying going on. It usually refers to anime original content that is adapted in-between source canon content.
The original content, the Filler, is seen as deviating from the source canon, even if it's part of the anime canon. It's called filler, because it rarely moves things forward, as it is spliced in and can't change much without disrupting following things from the source canon. E.g. a filler can't really kill off characters that are used later in the source stories.
That's why fillers are often disowned by source readers and disliked in general. Exceptions exist as always. People can barely tell apart what's manga canon in Gintama and what was added originally for the anime.

Example: Naruto filler list

How do I avoid anime filler?

There are a variety of ways to avoid anime filler:

1. Use Simkl's Fillers tab in anime that have one and check the episodes numbers that are marked as a filler.
2. Avoid watching anime that uses filler as a means of filling time, especially if you're not watching for enjoyment.
3. If an anime you are watching is not intended for you to watch, it will not have filler. However, in order to enjoy a piece of work, you must be willing to give it a chance.
4. Avoid watching anime that is full of action. If a series is meant to be an action show, it will not have filler.