A customer in a shop’s database might have the ID 42. In the customer list, this is a primary key: no other customer has the same ID. An order can store 42 as a foreign key, showing who placed it.

Why use IDs instead of names?

Names and email addresses can change or be shared. A separate ID keeps the link stable. If the database checks the link, it can reject an order that points to a customer who does not exist.