It's a 'container' file for images that is supposed to be high quality yet use a reduced file size. It's an Apple thing...
Also know as "High Efficincy File Format", HEIF
(this is what is inside of a HEIC).
Images sent between iPhones using text use it. The only way to get those images off of your phone is to mail it to yourself and then send it to a website that does conversion.
It is supposed to be better than JPG/JPEG but it is not widely supported. According to Wikipedia "As of March 2021, no browser supports HEIC natively."