What is the purpose of a primary key?

Primary key:

A primary key is the collection of field value which persist unique value in the table. The primary key value can be use to represent the entire record throughout the table. In the database, each table only contain one primary key.

The purpose of the primary key is used to implemented the relationship within the two tables in the database. More explicitly the primary key is define as, primary key is "target" that the foreign key can easily referenced. It basically consist multiple attributes in the table.