HEIC (based on the HEIF standard) is the format modern iPhones and iPads use to store photos. It packs the same quality as JPG into a much smaller file, which saves space on your device.
The problem is compatibility: many Windows apps, websites and older devices can't open HEIC files, so you often need to convert them to JPG before sharing or uploading.
You can convert HEIC to JPG instantly in your browser — the photo never leaves your device.
Why Apple switched to HEIC
HEIC is Apple's use of the HEIF container, built on the HEVC video codec. Applying video compression techniques to a still image roughly halves the file compared with JPG at the same quality, which is significant when a phone stores tens of thousands of photos. Since iOS 11, "High Efficiency" has been the default camera setting on iPhones.
Why it causes so much trouble
HEVC carries a complicated patent licensing situation, so support did not spread the way JPG's did. Windows requires a paid codec extension from the Microsoft Store to display HEIC in Explorer or Photos. Many websites reject the format on upload. Android support is inconsistent across manufacturers, and plenty of desktop software has never added it. The result is a format that works flawlessly inside Apple's ecosystem and awkwardly everywhere else.
What HEIC stores beyond the picture
A single HEIC file can hold more than one image — that is how Live Photos keep a still and a short motion sequence together, and how burst shots and edit histories are stored. Converting to JPG flattens all of that to one still frame. If the motion part of a Live Photo matters, export it as a video before converting.
Avoiding the conversion entirely
On the iPhone, Settings > Camera > Formats > Most Compatible switches the camera back to JPG permanently, at the cost of roughly double the storage per photo. There is also a separate setting under Settings > Photos > Transfer to Mac or PC that converts automatically when you transfer over a cable — which explains the common confusion of photos arriving as JPG one way and HEIC another.