No, it does not return an arraylist index.
The API documentation of Comparator.compare() says:
Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
What the actual number is doesn't matter. The only thing that matters is that it's < 0, == 0 or > 0.
The compare() method should only compare the two DVD objects that it gets passed as arguments. It doesn't know anything about the list that is being sorted; it doesn't even know that the DVDs are in a list. So it doesn't know anything about arraylist indices.