What does the '&' sign do when using 'auto' for iteration

recently I've encountered very very peculiar question when using auto in C++, just ... just look at the following code snippet :

my main function:

#include <list>
#include <iostream>
#include <stdio.h>
int main(){ 
    int a = 10, b = 20, c = 30;
    list<int> what;
    what.push_back(a);
    what.push_back(b);
    what.push_back(c);

    read(what);

    return 0;
}

And here's function read:

void read(const list<int>& con){

    for (auto it : con){
        printf("%p\n", &it);
        cout << it << endl;
    }
    return ;
}

And here's is the output :

0x7fffefff66a4
10
0x7fffefff66a4
20
0x7fffefff66a4
30

What the heck is that? Same address with different content !?

And more strange this is, if I modify the for-loop by adding an '&'
that is:

for (auto& it : con){

All the output makes sense immediately, the addresses would change by iteration

So my question is,
Why does the '&' sign make a change under this circumstance?