"as winds upon herbage": Just as winds descend upon herbage and enter into it so that it does not become wormy, you, too, "enter into" words of Torah so that you not forget them. Thus did R. Yaakov b. R. Chaninah say to Rebbi: Let us "enter into" the halachoth, so that they not become mouldy....
"and as showers upon the grass": Just as showers descend upon (blades of) grass, and cleanse them, and scour them, and expand them, so, expand words of Torah — a second time and a third time and a fourth time.