confused

[clipsit]

can anyone explain what is happening in this program I really dont understand it. thank you.

#include <stdio.h>

int main() {

  int index, freq[26], c, stars, maxfreq;

  for (index=0; index<26; index++)
    freq[index] = 0;

  while ( (c = getchar()) !=EOF) {

    if (isalpha(c))
      freq[2*(tolower(c)-'a')]++;
  }


  maxfreq = freq[0];
  for (index = 1; index < 26; index++) {

    if (freq[index] > maxfreq)
      maxfreq = freq[index];
  }

  for (index=maxfreq; index>=1; index--) {

    for (stars=0; stars<26; stars++) {

      if (freq[stars] >= index)
        printf("*");
      else
        printf(" ");
    }
    printf("\n");
  }
  printf("abcdefghijklmnopqrstuvwxyz\n");
system("pause");
  return 0;
}