It means post-Master's, hence usually doctoral studies.
Not really, doctoral is doctoral, and post-graduate is doing another MA (or BA) - not necessarily going up the ladder for PhD.
It means doing another licencjat or magister title, in other field.
but it isn't worth anything outside of Poland
Not quite true either. If you have more than one BA, BS or MA etc. and it is respected outside Poland (why wouldn't it be?) then you have specialities in those areas. Why wouldn't it be worth anything?