what is an include guard in c++ code example

Example: include guard c++

#ifndef PHYSICS_H
#define PHYSICS_H

#include "GameObject.h"
#include <list>


class Physics
{
private:
    double gravity;
    list<GameObject*> objects;
    list<GameObject*>::iterator i;
public:
    Physics(void);
    void ApplyPhysics(GameObject*);
    void UpdatePhysics(int);
    bool RectangleIntersect(SDL_Rect, SDL_Rect);
    Vector2X CheckCollisions(Vector2X, GameObject*);
};

#endif // PHYSICS_H