请教这个程序怎么理解,是干什么用的?

#include <stdio.h>

void main ()
{
printf("0 & 0 is %d\n", 0 & 0);
printf("0 & 1 is %d\n", 0 & 1);
printf("1 & 1 is %d\n", 1 & 1);
printf("1 & 2 is %d\n", 1 & 2);
printf("15 & 127 is %d\n", 15 & 127);
}

输出结果:
0 & 0 is 0
0 & 1 is 0
1 & 1 is 1
1 & 2 is 0
15 & 127 is 15