예제 푸는도중에 이해가되지않아 질문드립니다

#include <iostream>
using namespace std;
 
class Point 
{
protected:
    int m_x, m_y;
 
public:
    void Print() const;
    void SetXY(int x, int y);
    Point(int x = 0, int y = 0);
    bool operator==(const Point& p) const;
    Point operator+(const Point& p) const;
};
 
void Point::Print() const
{
    cout << "(" << m_x << ", " << m_y << ")";
}
 
void Point::SetXY(int x, int y)
{
    m_x = x;
    m_y = y;
}
 
Point::Point(int x, int y)
{
    m_x = x;
    m_y = y;
}
 
bool Point::operator==(const Point& p) const
{
    return (m_x == p.m_x && m_y == p.m_y);
}
 
Point Point::operator+(const Point& p) const
{
    return Point(m_x+p.m_x, m_y+p.m_y);
}
 
ostream& Point::operator<<(ostream &os, const Point& p)  //오류
{
    os << "(" << p.m_x << ", " << p.m_y << ")";
    return os;
}
 
int main()
{
    Point p1(10, 10);
    Point p2 = p1 + 20;
    cout << "p2 = " << p2 << "\n";
 
    Point p3 = 20 + p1;     //오류
    cout << p2 << "과 " << p3;
    if( p2 == p3 )
        cout << "은 좌표가 같습니다.\n";
    else
        cout << "은 좌표가 다릅니다.\n";
 
    return 0;
}