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.

It consists of information about a dataset such as its title and description, method of collection, author or publisher, area and time period covered, licence, date and frequency of release, etc. It is essential to publish data with adequate metadata to aid both discoverability and usability of the data.

Source: ODH, EU OD and NISO — http://www.niso.org/publications/press/UnderstandingMetadata.pdf