Metadata is image information stored alongside or within a photo. You don’t see it in the image, but it can be read by different software applications to help filter, sort, search or identify images.
A small data file stored alongside images by some photo-editing applications. The sidecar file contains processing data not embedded in the file itself, which is common practice for RAW files (which can't be modified). Sidecar files may also contain keywords and other metadata.