What is Metadata?
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 or information about information.
This definition comes from the National Institute Standards Organization (NISO) publication Understanding Metadata
Metadata in Plain Language - USGS gives a good overview for creating metadata
FINDABLE: Metadata and data should be easy to find for both humans and computers.
ACCESSIBLE: Once the user finds the required data, s/he needs to know how can they be accessed.
INTEROPERABLE: Data usually need to be integrated with other data. And data need to interoperate with applications or workflows for analysis, storage, and processing.
REUSABLE: Metadata and data should be well-described so they can be replicated and/or combined in different settings.