Metadata are data about data.
They help you find and use information. A library catalog entry gives you a book's title, author, subject, number of pages, publisher, publication date, and location in the library. It could be considered the book's metadata. More extensive metadata about the book might tell you what typeface was used, what kind of research the author did before writing the book, and how reliable you can consider it. When using geospatial data, there is a certain risk in just pulling a data set off the shelf without looking at the metadata.