Metadata is commonly described as "data about data." While easy to remember, this definition is far too vague to be useful. The definitions below provide better explanations in plain English.
Definition from the National Information Standards Organization (NISO)
"Metadata is structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource."
Definition from Steven Miller, Information and Metadata Lecturer
“Extra baggage associated with any resource that enables a real or potential user to find that resource and to determine value…”
Definition from Karen Coyle, Digital Librarian and Author of Coyle's InFormation
“Metadata is constructed, constructive, and actionable.”
As indicated in the Miller definition above, metadata helps people find resources and determine their value, for whatever need is at hand. This function of metadata is especially critical in digital environments, where humans rely on computer processing for reliable and timely results:
An interesting look at the importance of Metadata and why it really matters can be seen in this slideshow from Rachel Lovinger entitled "Metadata is a Love Note to the Future"