The meaning of srand()

I don't understand the meaning of srand() in <time.h> to create a random number. Here is my code:

/* srand example */
#include <stdio.h>      /* printf, NULL */
#include <stdlib.h>     /* srand, rand */
#include <time.h>       /* time */

int main ()
{
  printf ("First number: %d\n", rand()%100);
  srand (time(NULL));
  printf ("Random number: %d\n", rand()%100);
  srand (1);
  printf ("Again the first number: %d\n", rand()%100);

  return 0;
}

and the results are:

First number: 41
Random number: 13
Again the first number: 41

Why is the result of srand(1) different from srand(2)? Why does the result of srand(1) or srand(2) keep appearing again and again? And why do I have to use srand(time(NULL)) in order to create a dynamic random number?