Short Answer
Metadata summarize all necessary information about data in a structured way to make the data understandable and machine readable.
Detailed Answer
"Metadata is a love note to the future" (Jason Scott, @textfiles) ...to your future self, and to others who still want to make sense of your data in a few years.
Metadata can be considered as data about data and summarize all necessary information about data in a structured way to make the data understandable and machine readable. They should at least contain information on the data collector/owner, the content of the data, time and place of data collection, and the collection method - WHO? WHAT? WHEN? WHERE? HOW?.
See also:
- FAQ: How do I submit metadata and data?
- FAQ: Wat are descriptive metadata? Will they be archived together with the research data?
- FAQ: What is a metadata standard?