c pointers and pointers to arrays

I came across this piece of code on the net.I simply dont understand whats going on here.

#include <stdio.h>
#include <math.h>

main()
{
    int arr [ ][3] = {{11,12,13}, {21,22,23},{31,32,33},{41,42,43},{51,52,53}};
    int I , j ;
    int  *p ,  (*q) [3], *r ;
    p = (int *) arr ;
    q = arr;
    r = (int *) q ;
    printf ("  %u  %u  %d  %d  %d  %d \n  ", p, q, *p, *(r), *(r+1), *(r+2));
    p++ ;
    q++ ;
    r = (int *) q ;
    printf ("  %u  %u  %d  %d  %d  %d \n  ", p, q, *p, *(r), *(r+1), *(r+2));
}

i understand that p and r are pointers,q is a pointer to an array,but what does p = (int *) arr ; mean?What purpose does this statement serve?