C++ passing by reference or by value?

I am new to programming and here is a simple question about how passing by reference works. In this program, I am calculating roots of a quadratic equation.

void getCoefficients(double &a, double &b, double &c);
void solveQuadratic(double a, double b, double c, double &x1, double &x2);
void printRoots(double x1, double x2);


void error(string msg);

int main() {
        double a,b,c,x1,x2;
        getCoefficients(a,b,c);
        solveQuadratic(a,b,c,x1,x2);
        printRoots(x1,x2);
        return 0;
}

So, my question is I seem to be passing values to getCoefficients and solveQuadratic from main program but in the function definitions of getCoefficients and solveQuadratic, I seem to be accepting references as arguments and am confused as to how this works?