A HEIC file is a photo saved in the High Efficiency Image Container format, which iPhones and iPads have used by default since iOS 11. It stores the image with the HEVC codec at roughly half the size of an equivalent JPG at the same quality — great for saving space on your phone, but not supported by every app, website, or Windows PC, which is why the file sometimes won't open.
Why iPhones save photos as HEIC
Since iOS 11 in 2017, Apple made HEIC the default because it stores a photo at about half the size of a JPG at comparable quality. On a phone full of photos that adds up to gigabytes saved. HEIC also supports things JPG can't: higher colour depth (smoother gradients, less banding in skies), transparency, and multiple frames in one file — which is how Live Photos and depth data work.
What's the difference between HEIC, HEIF, and HEVC?
These names get used interchangeably but they're three different layers. HEIF (High Efficiency Image Format) is the container standard. HEIC is Apple's specific flavour of HEIF — the file with the .heic extension. HEVC (also called H.265) is the codec, the actual compression method used inside. So a .heic file is a HEIF container holding an HEVC-compressed image.
Why HEIC won't open outside the iPhone
HEVC is patent-encumbered and not supported everywhere. Windows can't open .heic out of the box, and plenty of websites, upload forms, older apps, and non-Apple devices reject it. That's why you email a holiday photo and the recipient can't open it, or a job portal refuses your .heic upload.
How to open or convert a HEIC file
You have two options. On Windows you can install Microsoft's HEIF Image Extensions from the Store (they depend on the HEVC extension, historically a paid listing). The simpler fix that works for everyone you send the file to is to convert it: HEIC to JPG fixes the file itself so it opens on any device, while HEIC to PNG gives you a lossless copy if you need to edit it. Both run entirely in your browser, so your photos are never uploaded.
Should you turn HEIC off?
You can — Settings → Camera → Formats → "Most Compatible" makes your iPhone shoot JPG instead — but you lose the storage savings. A better middle ground is to keep shooting HEIC and convert only the photos you actually need to share. Wondering whether that conversion costs quality? See does converting HEIC to JPG reduce quality, or compare the formats directly in HEIC vs JPG.