You send someone a photo from your iPhone and they reply that it will not open. You upload one to a website and the form rejects it. The file ends in .heic, and that is the whole story.
What HEIC actually is
HEIC is Apple's container for images encoded with HEVC, the same compression family used by modern video. It is genuinely better technology than JPEG: roughly half the file size at comparable quality, support for 10-bit colour, transparency, and multiple images in one file, which is how Live Photos work.
Since iOS 11, iPhones shoot in HEIC by default. That is why your photo library takes up less space than it used to, and also why sharing occasionally goes wrong.
Why so much software rejects it
The obstacle has been patent licensing rather than technical difficulty. HEVC is encumbered by patent pools, which made it awkward for browser and operating system vendors to ship decoders freely. Windows historically required a paid codec from the Microsoft Store. Many web upload forms simply never added it to their accepted list, and plenty of older editing software has no idea what the file is.
The result is a format that works flawlessly inside Apple's ecosystem and unpredictably outside it.
Fix one: convert the files you need to share
Convert to JPG for maximum compatibility, or PNG if you need lossless quality for editing. HEIC to JPG and HEIC to PNG both run in your browser, so holiday photos and personal pictures are not uploaded anywhere. For sending a batch as one document, HEIC to PDF is often more convenient than attaching twenty files.
Fix two: change what your phone captures
On iPhone, open Settings, then Camera, then Formats, and choose "Most Compatible". Your phone will shoot JPEG from then on. You lose the space saving and some quality at the same file size, and you stop having this problem.
A middle path worth knowing: leave the camera on High Efficiency and change Settings, Photos, Transfer to Mac or PC to "Automatic", which converts on export while keeping the originals compact on the device.
One thing to check before converting
HEIC files carry the same EXIF metadata as any other photo, including GPS coordinates, and converting to JPG usually preserves it. If you are about to share the result publicly, strip it first with remove EXIF data. See the EXIF guide for what it exposes.