what inline means?

My current thoughts.
  - if the function is not defined (no implementation) then inline will define the function.
  inline int f(){ return 1; }
  inline int f(){ return 1; } //error already defined
  - if the function is already defined somewhere then it is just a "view" of the code. (This view can be used to "inline code")
  extern int f();
  inline int f(){ return 1; } //ok
  - if a inline function is declared and not define at current file then it is a warning
inline int f();//warning