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.
This is an additional file saved by some photo editing software alongside an original image to store adjustments and/or keywords and other metadata. It's used by Adobe for RAW files, for example, where it's not possible to embed this data directly in the file. These XMP files are often called 'sidecar' files because they are saved separately to but alongside the image they relate to.