The following is a segment of code in an Application class of Hibernate. How come, this ME.addEmployee returns an integer?
Integer empID1 = ME.addEmployee("Zara", "Ali", 1000);
Integer empID2 = ME.addEmployee("Daisy", "Das", 5000);
Integer empID3 = ME.addEmployee("John", "Paul", 10000);

The following is the code for addEmployee;

public Integer addEmployee(String fname, String lname, int salary)
{
Session session = factory.openSession();
Transaction tx = null;
Integer employeeID = null;
try
{
tx = session.beginTransaction();
Employee employee = new Employee(fname, lname, salary);
employeeID = (Integer) session.save(employee);
tx.commit();
}catch (HibernateException e)
{
if (tx!=null) tx.rollback();
e.printStackTrace();
}
finally
{
session.close();
}
return employeeID;
}

Your question is somewhat unclear to me.
Looking at the code u are using session.save which returns the the id of entity persisted in the database.

Initially the values in databases are null then how session.save can return the id?
I think session.save saves the details of the employee(fname, lname, salary) into the database.

Its all confusing. can anyone tell me what session.save() actually does?

In layman if your generator class is native/sequence/etc then its hibernate job to save the entity in database with a valid key.
Once it save in database it returns the primary key /id that it used earlier.
In case generator class is assigned it will return the same key /id provided you..

Hope its clear now