Iterative Quicksort steps?

Question

I have to implement an iterative quicksort in java for homework. i've search a lot about it but i couldn't find a website with a clear explanation about how to implement an iterative quicksort.

i have found this code in java, and it sorts very well, but i can't tell how does it work, i know how the recursive quicksort works though.

i've commented the code with my questions

public static void iterativeQsort(int[] arr) { 
    Stack<Integer> stack = new Stack<Integer>();
    stack.push(0);
    stack.push(arr.length);
    while (!stack.isEmpty()) {
        int end = stack.pop();
        int start = stack.pop();
        if (end - start < 2) continue;
        int p = start + ((end-start)/2);
        p = partition(arr,p,start,end); 

        stack.push(p+1);
        stack.push(end);

        stack.push(start);
        stack.push(p);

    }
}

private static int partition(int[] arr, int p, int start, int end) {
    int l = start;
    int h = end - 2; //what is h and l variables for and why h has to be end -2?
    int piv = arr[p];
    swap(arr,p,end-1);

    while (l < h) {
        if (arr[l] < piv) {
            l++;
        } else if (arr[h] >= piv) { 
            h--;
        } else { 
            swap(arr,l,h);
        }
    }
    int idx = h;  // what is idx exactly?
    if (arr[h] < piv) idx++;
    swap(arr,end-1,idx);
    return idx;  //why return idx.
}
private static void swap(int[] arr, int i, int j) { 
    int temp = arr[i];
    arr[i] = arr[j];
    arr[j] = temp;
}

i'm mostly confused by the partition method, i don't know what it does.

if someone could explain me the main steps to make an iterative quicksort i'll be very happy.

Thanks for your help.