c++ const in the argument

why this code shows error while compiling?

#include <iostream>

using namespace std;

void foo(int& x){
   // cout<<x;
}

int main(){
    //int x=3;
    foo(3);
    return 0;
}

but by changing the argument to const it compiles properly

#include <iostream>

using namespace std;

void foo(const int& x){
   // cout<<x;
}

int main(){
    //int x=3;
    foo(3);
    return 0;
}

but i am still passing an integer so how does it compiles by adding const in the argument?