A Primary Key (PK), a term that comes from data modeling, defines a column, or columns, that uniquely identify a row in a table. Because it is used for identification, a PK cannot be null. There must be something in that column, or columns, that uniquely identify it. Moreover, PK values should not be changed. Historical information, as well as relationships with others tables, may be lost if a PK is changed or re-used.
A PK is a logical relational database concept. It may or may not be the best column, or columns, to choose as a PI for a table.