What Does It Mean For a C++ Function To Be Inline?

// my_thing.h
inline int do_my_thing(int a, int b) { return a + b; }

// use_my_thing.cpp
#include "my_thing.h"
...
    set_do_thing(&do_my_thing);

// use_my_thing_again.cpp
...
    set_other_do_thing(&do_my_thing);

use_my_thing_again.obj : error LNK2005: "int __cdecl do_my_thing(int,int)" (?do_my_thing@@YAHHH@Z) already defined in use_my_thing.obj
<...>Scratch.exe : fatal error LNK1169: one or more multiply defined symbols found

int foo();
inline int bar();

int foo(){ int r = bar(); return r; }


 inline int bar(){ return 5;};

#include "inlinetest.h"
 int main()
 {
  foo();
 //bar();
  }