Why this operation is called autoboxing?

I have this code:

int[] a;
Random rand = new Random(47);
a = new int[rand.nextInt(20)];
for(int i = 0; i<a.length; i++)
    a[i] = rand.nextInt(500); //Autoboxing 

The book calls the last instance 'Autoboxing' but it doesn't explain the reason.

Could tell me why?

Thanks