A constructor is a special member function that runs automatically when you create an object. It sets up initial values for member variables, so you don't start with garbage data. You write it with the same name as the class and no return type: Point() { x = 0; y = 0; }.
Now every Point object starts at the origin. Without a constructor, member variables hold random values until you manually assign them. Constructors enforce that objects always start in a valid state.