why does it print out random characters in c++ code example

Example: getting a random letter in c++

char c;
    int r;

    srand (time(NULL));    // initialize the random number generator
    for (i=0; i<num; i++)
    {    r = rand() % 26;   // generate a random number
          c = 'a' + r;            // Convert to a character from a-z
          cout << c;
    }