Metadata describes data. Data in here means something useful to the users. Metadata helps the data consumers (library users, staff of library or IT dept) achieve certain purposes. But metadata itself can be a kind of data.
Read this interesting article “Metadata, So Mom Can Understand” to know more.
Our bibliographic metadata (e.g. cataloguing records) does not only help users to find the resources in our collection, it can be ‘data’ in the context of Semantic Web. This is one of the arguments supporting the Linked Library Data movement.