In general, "Metadata is structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource. Metadata is often called data about data or information about information." (Understanding Metadata, National Information Standards Organization, 2004) A familiar example of metadata is that which describes books: it can include the author, title, publisher, edition, date and place of publication, illustrator, type of illustrations, number of pages, and much more. Metadata about an electronic file could include author, file type, date file was created, update dates, file size, type of contents, etc. Metadata helps researchers understand the item they are encountering.
There are numerous metadata systems or standards that a researcher can apply to suit their own needs.