Difficulty in NULL concept in C?

NULL in C programming can anyone tell me how NULL is handled in C?And the output of this program is 3, how with NULL concept?

#include <stdio.h>int main(void) {    int i;    static int count;    for(i = NULL; i <= 5;) {        count++;        i += 2;    }    printf("%d\n",count);    return 0;}