AVIF is a modern image format based on the AV1 video codec. Websites use it because it makes images small, so when you save a picture from a web page you often end up with a .avif file. Many photo editors, office programs, upload forms and older phones still can't open AVIF, while JPG works everywhere.
This converter decodes each AVIF in your browser (using the browser's own decoder, or a WebAssembly decoder where the browser has none) and saves it as a JPG at the quality you choose, 90% by default. JPG has no transparency, so see-through areas, common in AVIF logos and product cut-outs, are filled with white or black, your choice. You can also scale images down to 2048, 1600 or 1080 px on the long edge.
Limits: AVIF is usually more efficient than JPG, so the JPG will often be larger than the AVIF it came from. HDR and wide-gamut AVIF images are converted to standard colour. Animated AVIF files are treated as still images, so you get at most a single frame.