In-between chapter content is usually just "bonus" and can be anything you want! In addition to backstage sneak peaks like mentioned above, it can be a place to put stuff that wouldn't fit in the main comic such as:
- Gags and fun jokes that happened while Serious Action was underway
- Character bios/sketches
- Worldbuilding textwalls
Or just things that are tangental to the comic:
- Art you drew, AUs,
- Holiday-themed greetings
- Style changes, anime/tv show tributes
- Q&As, either written by you (if you want to only have specific questions answered, similar to character bio) or from your readers
And that's just off the top of my head! And, of course, you don't have to do any of that if you don't want to. If it's not fun, don't bother 