Metadata can be defined as "data about data". In particular, there are descriptive metadata (which refer to the data set) and structural metadata (which describe the organizational structure of the data). Good practices on metadata are included in the document produced by Research Data Canada: Guidelines for the Deposit and Preservation of Research Data in Canada.
- Dataverse North Metadata Best Practices Guide (Portage & CARL)
A Readme file provides information about how to interpret a data file. Without it, there is a good chance the data will be misunderstood and misused by other researchers.
A data dictionary contains definitions and descriptions of variables, ranges of accepted values for variables, units of measure, etc., thus allowing other researchers to understand your data.