can any body help to understand this java program

it just print prime n.o ,as you enter how many you want and it works good

class PrimeNumbers
{

public static void main(String args[])

{

int n, status = 1, num = 3;


 Scanner in = new Scanner(System.in);

  System.out.println("Enter the number of prime numbers you want");

   n = in.nextInt();



      if (n >= 1)
      {
         System.out.println("First "+n+" prime numbers are :-");
         System.out.println(2);
      }

      for ( int count = 2 ; count <=n ;  )
      {
         for ( int j = 2 ; j <= Math.sqrt(num) ; j++ )
         {
            if ( num%j == 0 )
            {
               status = 0;
               break;
            }
         }
         if ( status != 0 )
         {
            System.out.println(num);
            count++;
         }
         status = 1;
         num++;
      }         
   }
}

i dont understand this for loop condition

     for ( int j = 2 ; j <= Math.sqrt(num) ; j++ )

why we are taking sqrt of num...which is 3....why we assumed it as 3?