Where is the logic in flag in finding prime number

#include <iostream>
using namespace std;
int main()
{
int n, i, flag=0;
cout << "Enter a positive integer: "; cin >> n;
for(i=2;i<=n/2;++i)
{
if(n%i==0) {
flag=1;
break;
   }
}
if (flag==0)
cout << "This is a prime number"; else cout << "This is not a prime number";
return 0;
}

Code (markup):

 

Everything I post here is not a home work. I'm just learning programming and I don't want to cram. What I know about prime is the number which can be divided by one or itself. The first condition I have already understood even number can't be prime number when reminder is zero.
More I want to know when flag is set equal to one. How does the program detect that number can be divided with one or itself