Complex sentence: Orach Chayim is a section of Rabbi Jacob ben Asher's compilation of Jewish Law, Arba'ah Turim, that treats all aspects of Jewish Law primarily pertinent to the Jewish calendar (whether the daily, weekly, monthly, or annual calendar).
Split into simpler sentences: Orach Chayim is a section of Rabbi Jacob ben Asher's compilation of halakha (Jewish law), Arba'ah Turim. This section treats all aspects of Jewish law primarily pertinent to the Jewish calendar (whether the daily, weekly, monthly, or annual calendar).