What is the meaning of "Lehoros be'Yom ha'Tahor u've'Yom ha'Tamei"?
Rashi: It means to teach the laws concerning days that the Metzora is Tahor and days that he is Tamei. 1
Targum Yonasan: It means that the Kohen should teach people the difference between a cloudy day on which one cannot examine Nega'im and a bright day, on which one can.
See Sifsei Chachamim.
Why does the Torah see fit to add "Zos Toras ha'Tzara'as?
Seforno: To issue a prohibition against adding any appearance that does not match those listed in the Torah. 1 which are Tahor just like a Nega that spreads to the entire body.
Moshav Zekenim (12:8, citing Vayikra Rabah 16): In five places it says 'Toras' next to Tzara'as (also 13:59, 14:2, 32, 54), to teach that because instead of engaging in the five Chumashim, he engaged in Leshon ha'Ra 2 , which is equivalent to all the Aveiros in the Torah, Tzara'as came on him. 'Metzora' is like Motzi Ra, i.e. saying bad [things about people].
Moshav Zekenim citing R. Yehudah ha'Chasid: Whenever it says "Zos Toras", the entire matter was explained here. You will not find more anywhere, even in Mishneh Torah.