HEIC (based on the HEIF standard) is the image format Apple devices use by default. It stores photos at roughly half the file size of a JPG at the same quality, saving space on your phone.
The catch is that many non-Apple apps, websites and older programs can't open HEIC, so you often need to convert iPhone photos to JPG to share them.
Convert with HEIC to JPG — free and in your browser.