A file format for photos that's designed to be more efficient (in terms of file size) and higher-quality (in terms of visual detail) than the JPEG and GIF standards it's intended to replace.
Like JPEG, HEIF is a lossy compression format, meaning some visual detail is permanently lost when creating the HEIF file. This permits a smaller file size, using less storage space. However the intention of HEIF is to apply this compression in a way that the average human eye can't notice.
Unlike JPEG, HEIF files can include animation.