Question

what is a primary key?

Answer 1

If you are asking for a pc game, then it is the key registered by the game to which it is more responsiveif more than one keys are pressed.

Answer 2

Primary key shoud not allowed null and repeated values,student id and employee id best example for promary key.
    A table should not have more than one primary key .it should have only one primary key. 
   But combination of more than one columns can be populate one primary key.