win32:LIBS = $$member(LIBS, 0) $$member(LIBS, 1)d

While studying the Plug & Paint Example I got stuck at the project file, more specifically at the following:

if(!debug_and_release|build_pass):CONFIG(debug, debug|release) {
   mac:LIBS = $$member(LIBS, 0) $$member(LIBS, 1)_debug
   win32:LIBS = $$member(LIBS, 0) $$member(LIBS, 1)d
}

What this peace of code does? What's $$member(LIBS, 0)?