A MetaObject is a class that implements a programmer's interface for a VOS type. Vobjects allow you to query a MetaObject using a type name or MetaObject class name, giving you an object with which you can more easily interact with an object of a specific type.
This is the main way you can extend VOS for your specific data or application.
For some initial documentation on the MetaObjects that are included with VOS, see MetaObjectDocumentation.