What does Psalm 90:6 mean?

Moses compares human life to grass that looks healthy for a brief time but quickly loses its color and withers. Hot, dry climates can wreak drastic changes on plants. A stand of grass may be full and verdant in the morning, but when the hot sun appears the grass quickly pales and dries up.

The prophet Isaiah makes a similar comment about human life resembling grass. He writes: "A voice says, 'Cry!' And I said, 'What shall I cry?' All flesh is grass, and all its beauty is like the flower of the field. The grass withers, the flower fades when the breath of the LORD blows on it; surely the people are grass" (Isaiah 40:6–7). The apostle James comments on the brevity of life in James 4:14 by comparing it to a mist that appears for a brief time but then vanishes. In view of life's brevity and uncertainty no one should boast about what he will do tomorrow (James 4:13–16; Proverbs 27:1).