Every file, whether a text, photo, movie, and so on, will have attached "metadata", or data about that file. There are many data values that can be associated with a file, such as a title, subject, ratings, tags, comments and MANY others. They are "invisible" normally, but can be seen and edited by going to the "Details" tab of the file properties.
Why consider using "metadata"? Searching for specific files is greatly enhanced when searching for specific metadata, especially if you have thousands of files.
Programs like Adobe LightRoom, darkTable, Adobe Bridge, ACDSee, and other can help with adding metadata to your files, especially in large "batches".