A JXL extension on a file name indicates that the file is a JPEG XL image file. In supported applications, you can use the file in the same way you would use a JPEG or PNG file.
AVIF is another modern image format that competes with JPEG XL. While AVIF is also an improvement upon the legacy image formats, it is generally less advanced when compared to JPEG XL.
You can start using JPEG XL on websites right now! The HTML <picture> element ensures backward compatibility by allowing you to specify multiple image sources - browsers that support JPEG XL will use the JXL version, while others will fall back to legacy formats like JPEG or PNG.