Metadata is often, and most simply defined as "data about data," meaning that it uses pre-determined, often standardized terms or codes to describe the data including how it was collected, when it was collected, who collected the data, definitions of variables, how data relates to one another, and other relevant or pertinent information for understanding the scope, purpose, and provenance of the data, etc.
The National Information Standards Organization (NISO) defines metadata as "structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource."
For more in-depth introductions to metadata, see the links below:
Metadata is useful for:
Plan to include documentation on your methodology, analysis, a data dictionary, and hardware and software used.
The Research Data Alliance has a metadata standards catalog you can use to search for schema relevant to your discipline.