"= default" about constructor in c++ [duplicate]

This question already has an answer here:

• What does “default” mean after a class' function declaration? 3 answers

struct Sale{
Sale () = default;//I got nothing about those cod
Sale(std::istream &input);
Sale(const std::string &s):book(s) {}
//and so on......
}

What did the second code mean? Is that mean that doing nothing ?Or we must write those default code to tell the compiler we made a default constructor which doing nothing. And what about the "= default" mean in the C++11?