What is a Metadata? 

Metadata is information about a PE. In COM, metadata is communicated through non-standardized type libraries. In .NET, this data is contained in the header portion of a COFF-compliant PE and follows certain guidelines; it contains information like the assembly's name, version, language (spoken, not computer-a.k.a., "culture"), what external types are referenced, what internal types are exposed, methods, properties, classes, and much more. The CLR uses metadata for a number of specific purposes.

Security is managed by a public key in the PE's header. Information about classes, modules, and so forth allows the CLR to know in advance what structures are essential. The class loader component of the CLR uses metadata to locate specific classes within assemblies, either locally or across networks. Just-in-time (JIT) compilers use the metadata to turn IL into executable code.