What does an ampersand after this assignment operator mean?

I was reading through this nice answer regarding the "Rule-of-five" and I've noticed something that I don't recall seeing before:

class C {
  ...
  C& operator=(const C&) & = default;
  C& operator=(C&&) & = default;
  ...
};

What is the purpose of the & character placed in front of = default for the copy assignment operator and for the move assignment operator? Does anyone have a reference for this?