Metadata is often described as 'data that describes data'. You're already familiar with a number of kinds of metadata.
Library catalogues are a kind of metadata.
In and of themselves, they only contain information about books. They don't actually contain any information about the subject of a book. In MS Word, a kind of metadata is stored in documents in the form of 'document information'. The sort of metadata you'd want to record about a document would depend on the kind of document. For example, imagine you wanted to describe all the books at your home.