High Efficiency Image File Format (HEIF) is a container to store an image or a sequence of images in a single file. It is designed to be extremely lightweight and efficient.
High Efficiency Image Coding (HEIC) is the variant that Apple uses in its iOS and macOS operating systems.
The performances (smaller size, better quality) of these formats are often superior to other image formats like JPEG and JPEG 2000, especially for large pictures.
Since .heic adoption by Apple as default image storage format in 2017 (with the introduction of iOS 11) to gradually replace JPEG, support on other platforms and devices is growing.
However, there is still room for improvements with browsers and online applications.