Metadata is information that describes, explains, locates, classifies, contextualizes, or documents an information resource. It is what enables you to search for books in your local library catalog, videos on YouTube, or find journal articles through PubMed. It is also what can help manage data, by tracking attributes like data provenance and versioning. Metadata can be used to describe all types of information sources.
Metadata in libraries is generally structured as a set of metadata elements. “Creator name,” “Date created,” and “Keyword” are all frequently employed metadata elements. A metadata schema is a formalized collection of required and optional metadata elements that can help standardize how people and institutions describe information resources. Employing a metadata schema can help ease the process of searching for resources and sharing information about resources.
This resource from NISO provides introductory information on metadata, including basic information on how it is structured and what it is used for.
NISO: Understanding Metadata: https://www.niso.org/publications/understanding-metadata