What is a HEIC file?
HEIC is the format iPhones and iPads use for photos by default. It stores photos efficiently, often in less space than a JPG of similar quality. But support outside Apple devices is uneven: some versions of Windows need an extra extension, many websites and upload forms reject HEIC, and some photo apps can't open it.
JPG is the opposite. It is accepted almost everywhere, which is why converting is the simplest fix.
How to convert HEIC to JPG
Upload the HEIC file. Most browsers can't display HEIC, so the original side of the compare slider shows a placeholder, but the converted JPG appears as soon as it is ready. Set the quality, where the default of 90 percent keeps the photo looking the same, then download.
The photo is decoded on the server, turned upright according to its orientation tag and saved as a standard JPG. Its color profile is kept, so colors look as they did on your iPhone in apps that support color management.
What about metadata, Live Photos and HDR?
EXIF data such as the GPS location, date and camera details is not copied into the JPG, which is useful when sharing photos publicly. Live Photos and bursts give you the main still image. HDR brightness information isn't carried over, so you get a standard JPG suited to any screen.
Getting JPGs from your iPhone directly
If you'd rather get JPGs from the start, iPhones have a setting under Settings, Camera, Formats, where Most Compatible saves new photos as JPG instead of HEIC. Depending on your settings, photos may also be converted automatically when you transfer them to a computer. For files that are already HEIC, this converter does the job in one step.