Its a name-decoration for the stdcall calling convention, when making the DLL some compilers allow you to remove it or redefine the name without it.
Edit: Most times Extern "Win32" will get the name right anyway, you really only need the "GetEnvironmentVariableA@12" if you want to name the function something different.
Edit: Most times Extern "Win32" will get the name right anyway, you really only need the "GetEnvironmentVariableA@12" if you want to name the function something different.