Example in blue book:A car.
A car (entity, identified by vehicle identification number) have 4 wheels (entity, identified by left front, left rear , right front, right rear).
I dont need to (and want to) know how the wheels works when I drive a car. The car entity encapsulates them for me as an aggregate root.
Another example in blue book: A purchase order.
At any time, the total amount of sub-purchase orders(local entity) can not be greater than the limit of the purcahse order(aggregate root in this case). The aggregate root should check constraints all the time.