Moses likens the words of his song to rain falling on grass. The Sages interpret this as relating to the Torah in general, and explain why it is likened to rain.

“My lesson will fall as rain” (Deuteronomy 32:2). Just as rain falls on the trees and contributes flavors to each and every one of them according to its species – to the vine according to its species, to the olive tree according to its species, to the fig tree according to its species – the same is true for matters of Torah: It is one entity, yet it includes Bible, Mishna, Talmud, halakha, and aggada.

“Like rainstorms on grass” (32:2). Just as rainstorms fall on vegetation and cause it to grow tall, some red, some green, some black, and some white – the same is true for the Torah: It affects people, and the result is that some are wise, some are honest, some are righteous, and some are pious. (Sifrei Devarim, Haazinu 506)