What does the "&" that precedes the "=" in the assignment operator declarations do [duplicate]

This question already has an answer here:

• What does the single ampersand mean here? 3 answers

I was going through an question What does “default” mean after a class' function declaration? and operator declarations like

  C& operator=(const C&) & = default;
  C& operator=(C&&) & = default;

I wanted to know what does the "&" that precedes the "=" in the assignment operator declarations do?