This permission is for the ability to read the name and type information of non-public members of types (but not actual access to contents of members). Also, ability to find subclasses and superclasses, what module and assembly the class is in. For more details, see ReflectionPermission API specification.
The ability to use Reflection.Emit is also controlled by this permission. Reflection.Emit has important security implications as it allows code to create dynamic assemblies. The ReflectionEmit permission should be granted with caution.