Literally, it means unreasoning hatred of semites, and things semitic. Semite/semitic being a language group that includes aramaic, hebrew and arabic. This would thus include Arabs.
However, in accepted usage, it means hatred of Jews, Judaism, things Jewish.
Thank you very much for the explanation, however, If antisemitism in its worst case is considered to be a mere 'sin', why is it also considered a blunder in politics?