AVIF is an image file format like JPEG, PNG, or WebP but newer and better. AVIFs look very colorful and almost identical to the original picture while being much more optimized in file size than even WebPs, but are not very efficient on slow devices.
AVIF was released in 2020 and WebP was released in 2010 both first in Google Chrome, while PNG was released in web browsers in 1998 and JPEG goes back to the 1980s I think. Firefox didn't add support for AVIF until 2021. Apple didn't support it until 2022, because it was just finalized the other year and Apple wanted to make it work across their entire OSes, not just Safari.
The AVIF versions should have more vivid color and smoother graphics. I recommend the AVIF format only for very new and relatively high end devices as it requires more processing power / battery life.
Many current e-book readers like the Amazon Kindle are running software and hardware that is similar to that of the Palm Pilots of the 1990s and so cannot handle the newer file formats like WebP (and definitely not AVIF) and maybe still not even PNG or SVG.
Most Apple Users
If you have Apple iOS 16 (on iPhones and iPads) or MacOS 13 or higher, you have complete support for AVIFs (probably also including AVIFs in EPUB (e-book) format). Those looking to open AVIFs in iPhones, iPads, or Macs (not updated with free software (OS) updates since fall of 2022) can still see them by using the WebP Zip file format version.
Other Apple Users
Apple iOS updated since fall of 2019 and Windows and Macs since around 2005 support opening Zip files without additional software. The individual pictures need to be opened with some App Store app for iPhone and iPad users (before iOS 16) that supports the AVIF format. GraphicConverter 11 supports AVIF.
Windows and Other Users
These individual pictures can also be opened in Google Chrome and Firefox on any device that runs them except iPads and iPhones. Windows 10 users can also open them in Microsoft Paint.NET commercial version or in Microsoft Paint if they download an extra free download called AV1 Video Extension from the Microsoft Store. Android (and probably Linux) users should have even better support for this format than Apple or Microsoft users. Gimp supported AVIF since 2.10.22. Nomacs, Eye of Gnome, gThumb, Gwenview, and digiKam also support AVIF.