In a relation or table can have more than one choice of primary key, but only one of them set as a primary key. All the others keys which are able to declared as primary key are called an Alternate Key. That is all alternate key of a table can uniquely identify every tuple in that relation or table.
Lets consider, a relation called emp
In the above table Emp_SSN and Emp_No both can identify a student uniquely. We can declare those fields as a primary key. That is why those fields are known as alternate keys