RobH_en_tn/isa/40/07.md

when the breath of Yahweh blows on it

Possible meanings are 1) "when Yahweh blows his breath on it" or 2) "when Yahweh sends a wind to blow on it."

humanity is grass

The speaker speaks of humans as if they are grass, because they both die quickly. Alternate translation: "people die as quickly as grass" (See: rc://en/ta/man/translate/figs-metaphor)