Pass pointer as parameter in C function

I read all the answer questions on this topic and none of them answer my question...

I'm still reading about pointers in C, and now I'm trying to understand how to pass pointers through functions. However, the following code (taken from tutorialspoint.com) has something I don't understand:

#include <stdio.h>

/* function declaration */
double getAverage(int *arr, int size);

int main () {

   /* an int array with 5 elements */
   int balance[5] = {1000, 2, 3, 17, 50};
   double avg;

   /* pass pointer to the array as an argument */
   avg = getAverage( balance, 5 ) ;

   /* output the returned value  */
   printf("Average value is: %f\n", avg );
   return 0;
}

double getAverage(int *arr, int size) {

   int  i, sum = 0;       
   double avg;          

   for (i = 0; i < size; ++i) {
      sum += arr[i];
   }

   avg = (double)sum / size;
   return avg;
}

When calling getAverage(balance, 5), I pass in a pointer to the first element of the balance array (as I learned in the previous question). But how do we access the actual content of the array (sum += arr[i]) in the for loop in getAverage?

I read the following resource: http://c-faq.com/aryptr/aryptrequiv.html and I 'm pretty sure the sentence after the second footnote explains this. But I still don't get it.

Any help is appreciated!