Keys

Identity and reference.

Primary keys

A primary key identifies a row uniquely and permanently. A generated id is usually better than a natural value, because natural values such as emails and phone numbers change.

Sequential integers are compact; UUIDs can be generated anywhere without coordination.
A key should never carry meaning that might need editing.

Foreign keys

A foreign key points at another table's primary key and can be enforced by the database. That enforcement is the difference between a relationship and a hopeful convention.