Rashi: It is referring to "ha'Devarim asher Ra'u Einecha" in the previous Pasuk - 'Do not forget the things that your eyes saw, the 'voices' and lightning that were visible on the day that you stood on Har Chorev ... '.

Targum Yonasan: With reference to the Mitzvah of learning and teaching Torah mentioned in the previous Pasuk, the Pasuk is coming to forbid a Ba'al Keri (one who had a seminal emission) from learning and teaching Torah, ¹ just as Yisrael were all Tahor on the day that the Torah was given.