A common definition is that metadata are data about data. Metadata are all around us all the time, but when it does it’s job well, it is invisible. As an example let’s consider a book. Metadata are the information that describes the book (i.e. author, publisher, language, publisher, shelf number etc.). Metadata are descriptive of the book.
Let’s consider a youtube video. On a youtube page we have the actual video content and around it a great deal of metadata, like: title, creator, date uploaded, description of video, category, comments, related video… All these are information about the video but not the video itself. We can identify different types of metadata, from different processes: those created by the creator/uploader of the video (i.e. narrative description (free to write whatever), category and license (selected from a firm list of possible options)), those created by other people (commends) and those created automatically by youtube according to a special computer algorithm (related videos).
A better definition is that metadata is a description. But a description of what? What does it mean to describe something?