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.