How is this valid C++? What are these tuples supposed to be?

Question

How is (1,2,3) interpreted by the compiler? What C++ feature is this?

#include <iostream>
using namespace std;

int main() 
{
    int x = (1,2,3);
    cout << x;
    return 0;
}

https://onecompiler.com/cpp/3xrfgja4b