Can anyone explain how it really works..

Question

this type of question came a lot of time in challenge quizes.I didnt fully understand it..
#include <iostream>
using namespace std;

int main() {
int arr[]={1,2,3
,4};

for(int i=0;i<3;i++){
arr[1]= arr[i];
cout <<arr[i]<<endl;
}
}

Answer 1

which part don't you understand