A primary key is a special index which uniquely identifies a record, like enrollment number of a student identifies him in a university, account number in a bank etc etc.
Please see the characteristics in detail -
A primary key is implicitly UNIQUE - you cannot have more than one row with the same primary key, since its purpose is to uniquely identify rows.
A table can have only one primary key, every table should have one (not necessarily).
A primary key can never be NULL, so the row(s) it consists of must be NOT NULL.