An aggregate is an encapsulation of entities and value objects (domain objects) which conceptually belong together.
It also contains a set of operations which those domain objects can be operated on.
As a concrete example, an aggregate might be a Car, where the encapsulated domain objects might be Engine, Wheels, BodyColour and Lights; similarly in the context of manufacturing a car, operations might be: PaintBody, InstallWheel, InstallEngine and InstallLight and Ship.