About this tool
HEIC is the format the iPhone has used since iOS 11 to save photos at a smaller size. The problem is that Windows, many websites and several programs can't open it, so it's worth converting them to JPG.
This tool decodes the HEIC on your device, so your personal photos aren't uploaded to any server. The first conversion may take a few seconds because the decoder has to load.
If you'd rather have your iPhone save in JPG from the start, go to Settings › Camera › Formats and choose "Most Compatible."