Recursive program to find vowels

[DocCeaser]

This program uses recursion to find the number vowels in a string. Can someone please explain to me what the purpose of "subscript" is in the program. The program compiles and runs perfectly.

#include <iostream>
#include <string>
using namespace std;
int vowels( string, int);
bool isVowel(char);
int main()
{
string array;
cout<<"Enter a string: ";
getline(cin,array);
cout << "The are "<< vowels( array, 0) << " vowels in \""<<array<<"\"\n";
system("pause");
return 0;
}
int vowels( string str, int subscript)
{
if (str[subscript] == '\0')
{

return 0;
}
else
  if (isVowel(str[subscript]) )
{

return 1+vowels( str,subscript+1);
}
else
{

return vowels( str, subscript+1);
}
}
bool isVowel(char in)
{
     switch(in)
   { case 'a':
     case 'A':
     case 'e':
     case 'E':
     case 'i':
     case 'I':
     case 'o':
     case 'O':
     case 'u':
     case 'U':      return true;
   }             
 return false;                             
 }

[Moschops]

subscript defines which letter in the string is being examined.

[DocCeaser]

thank you.