Overview Diagram represents a logical structure of entities and the relationships among them. Each entity represents an object that is a natural part of and is important to the business and its business processes. Overview diagrams give you an overview of the relationships among these business objects.
How to model an overview diagram is explained Overview Diagram Reference in Developer Studio.
One example of a business object could be a customer order which in its simplest implementation could exist of the two entities Customer Order header and Customer Order lines. This business object has references to Customers and Sales Parts.