AVIF gives excellent compression, but plenty of image editors, older CMS platforms, and design tools still don't open it. Converting to PNG trades AVIF's small file size for universal compatibility and lossless quality, which matters if you need to edit the image further or embed it somewhere that only accepts common formats. The conversion happens by decoding the AVIF in your browser and re-encoding it as PNG, so the original pixels are preserved exactly (no re-compression artifacts beyond AVIF's own, already-baked-in lossy encoding, if the source used it).
Still images built out of a video codec
AVIF is a single frame of AV1 video wrapped in an image container. That sounds like a curiosity but it is the reason for its compression: modern video codecs have absorbed decades of work on predicting and describing image content efficiently, and a still image gets all of it. AVIF regularly produces files half the size of a comparable JPEG.
It is also why support arrived late and unevenly. Browsers handle it now, but desktop applications, older operating systems, and a good deal of professional and print software still do not, which is the usual reason to convert one to something older.
PNG as the safe landing place
PNG is lossless, so the conversion introduces no compression artefacts of its own and preserves the alpha channel if the AVIF had one. The output will be substantially larger than the AVIF, often several times so, and that is expected: you are trading the compression away for compatibility, deliberately.
The one real caveat is colour depth. AVIF can store 10 or 12 bits per channel with HDR, and while PNG does support 16-bit channels, the practical output path here is 8-bit. A standard-range image converts exactly. An HDR one is mapped down to the narrower range, which is usually imperceptible but can slightly flatten the extremes of a very high-contrast image.