What is ‘the inverse side of association’ in a mapping?

Let us consider an example in which a customer can have multiple orders and for every order there has to be a customer.

In OO world, customer is the owner of order. In SQL world, an Order has reference to customer id.

It is a bi-directional one to many mapping from customer to order.

The inverse side in this mapping is the owner of object. In this case customer is the owner or order. Since an order cannot exist without a customer. But a customer can exist without an order.

Also customer has no column to save order data. But an Order table can store customer id, which is used for mapping.