Use of reference operator & in returning from function in c++

I am getting confused here in the code:

#include <iostream>
#include <ctime>
using namespace std;
double vals[] = { 10.1, 12.6, 33.1, 24.1, 50.0 };

double& setValues(int i)//if i remove the &operator, i get an error, why ?
{
    return vals[i]; // return a reference to the ith element
}

int main()
{
    cout << "Value before change" << endl;
    for (int i = 0; i < 5; i++)
    {
        cout << "vals[" << i << "] = ";
        cout << vals[i] << endl;
    }
    setValues(1) = 20.23; // change 2nd element
    setValues(3) = 70.8; // change 4th element
    cout << "Value after change" << endl;
    for (int i = 0; i < 5; i++)
    {
        cout << "vals[" << i << "] = ";
        cout << vals[i] << endl;
    }
    return 0;
}

If i remove the & operator in the function

double& setValues(int i);

i get the error as 'l-value required as the left operand of the assignment' Is that means the value vals[i] should be placed in the right? and with & operator this part becomes l-value ?