An abstraction is like a container object and is used to a group entities. It is very useful when working with large data models. For example, an abstraction called Order Processing can be used to group the Customer, Product, Order and Ordered Product entities. By activating an abstraction, you can now work with just the Order Processing part of the model (four entities) rather than all entities in the model. Entities which are not part of the abstraction are not displayed in the diagram.
Once defined, an abstraction can be activated in order to simplify the model diagram view. In addition, it allows you to build only the objects contained within the abstraction, rather then the entire model.
For more details, refer to 8. Abstractions.