What is the use of the & in the CPP overload operator argument?

-sharpinclude <iostream>
using namespace std;

class Box
{
public:

    double getVolume(void)
    {
        return length * breadth * height;
    }
    void setLength(double len)
    {
        length = len;
    }

    void setBreadth(double bre)
    {
        breadth = bre;
    }

    void setHeight(double hei)
    {
        height = hei;
    }
    //  +  Box 
    Box operator+(const Box& b)
    {
        Box box;
        box.length = this->length + b.length;
        box.breadth = this->breadth + b.breadth;
        box.height = this->height + b.height;
        return box;
    }
private:
    double length;      // 
    double breadth;     // 
    double height;      // 
};
// 
int main()
{
    Box Box1;                //  Box1 Box
    Box Box2;                //  Box2 Box
    Box Box3;                //  Box3 Box
    double volume = 0.0;     // 

                             // Box1 
    Box1.setLength(6.0);
    Box1.setBreadth(7.0);
    Box1.setHeight(5.0);

    // Box2 
    Box2.setLength(12.0);
    Box2.setBreadth(13.0);
    Box2.setHeight(10.0);

    // Box1 
    volume = Box1.getVolume();
    cout << "Volume of Box1 : " << volume << endl;

    // Box2 
    volume = Box2.getVolume();
    cout << "Volume of Box2 : " << volume << endl;

    //  Box3
    Box3 = Box1 + Box2;

    // Box3 
    volume = Box3.getVolume();
    cout << "Volume of Box3 : " << volume << endl;

    return 0;
}

where overloaded operator function:

    //  +  Box 
    Box operator+(const Box& b)
    {
        Box box;
        box.length = this->length + b.length;
        box.breadth = this->breadth + b.breadth;
        box.height = this->height + b.height;
        return box;
    }

The parameter in

has a const Box& b, and what"s the use of &? I can run normally if I remove it. It doesn"t mean to take the address of the variable, is it? I can"t figure it out

and there is only one parameter here, but the Box itself also participates in running
Box3 = Box1 + Box2;
during the operation. Does the binary operator have a default parameter of itself, but you can write other parameters without writing it? add up the
three:
Box4 = Box1 + Box2 + Box3;. Should
be written like this:
Update: it is wrong to write the following. It has been tried. The + operator has at most one parameter. If the three operators add up, you should still use the above method. Just write the three additions on the outside

Box operator+(const Box& b,const Box& c)
{
    Box box;
    box.length = this->length + b.length + c.length;
    box.breadth = this->breadth + b.breadth + c.breadth;
    box.height = this->height + b.height + c.height ;
    return box;
}

Thank you

CPP c

Feb.27,2021

Box operator+(const Box& b) { Box box; box.length = this->length + b.length; box.breadth = this->breadth + b.breadth; box.height = this->height + b.height; return box; }