Meta-data is data about data. Meta-data is information (authorship, classification, date, URL, etc.) about an informational resource. It could be a document (such as a webpage), an image, a dataset, or another resource. Metadata is valuable in the storage and retrieval of information. Resources supported by good-quality, structured metadata are more easily discoverable.
For instance, most websites contain metadata to tell the computer how to lay the words out on the screen.