In its most basic sense, metadata is information about data, and describes basic characteristics of the data, such as
Who created the data
What the data file contains
When the data were generated
Where the data were generated
Why the data were generated
How the data were generated
Metadata makes it easier for you and others to identify and reuse data correctly at a later date.
Well-structured metadata not only supports the long-term discovery and preservation of your research data, but allows for the aggregation and simultaneous searching of research data from tens or hundreds or thousands of researchers.
This is why domain-specific repositories typically require highly structured metadata with your data submissions: it enables highly granular searches on their aggregated content. This in turn makes your data easier to find.