An association links one class to another to indicate a relation between them. For example, you have an association between
Order which (with no further information) indicates that:
A customer can have an order or maybe several orders,
An order can belong to a customer or maybe shared by several customers.
The diagram in your question gives an additional info: there is one and only one customer by order, and zero or more orders by customer, so the rules become:
A customer have zero or more orders,
An order belongs to one and one only customer.
While aggregation is used in the association between
OrderDetail, it's not used in the association between
Order. This is done to indicate the life cycle dependency. If an instance of the
Order is destroyed, all instances of
OrderDetail inside it would be destroyed too; on the other hand, the instances of
Order are independent of the life cycle of the
Customer instance, thus no aggregation here.
This doesn't mean that an order can exist without a customer: the 1 ↔ 0..* association explicitly forbids the orders which have no customers. The difference between a 1 ↔ 0..* association and the aggregation really happens on life cycle level.