Statements that appear in the interface portion of the class are the prototypes of the functions that provide the interface.
Without access to some documentation or the definitions of those functions, we cannot be certain just what the functions do.
However, we recognize the operators that are being overloaded.
We know how we expect them to behave.
A good OOP design causes the overloaded operators to perform as normally expected on data members of the class to which they apply.