Primary Key
In plain terms
A column (or set) that uniquely identifies each row in a table.
Going deeper
A primary key guarantees each record is uniquely and non-null identifiable, and is the main lookup handle. Every table should have one.
Under the hood
Auto-incrementing integers or UUIDs are common; it's automatically indexed and referenced by foreign keys.
Examples
id columns, UUIDs, composite keys, auto-increment, record identity.
Think of it like...
Like a unique ID number assigned to each person in a registry.