The NISO describes metadata as: "structure 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". 

In the context of research data management metadata is necessary for: 

• Finding data from other researchers to support your research
• Reusing data 
• Helping other researchers find and reuse your research data 
• Using your own data in the future 

Source: NECDMC Module 3

A data dictionary is a document that describes the data fields or elements. Data dictionaries play a vital role in data quality. They are also crucial to data reuse.