Leaves follows a single leaf through its whole life, from a thin new growth to a brown scrap on the ground. The speaker starts by saying there are two kinds of leaves, one that means leaving and one that is simply nice to look at, but by the end admits the two may not be so different after all.
Along the way, the poem plays with the phrase "green and young." People use "green" as a metaphor for inexperience, the speaker notes, while a leaf is literally green and only metaphorically young. Then comes decline: red color, hardened stems, fading beauty, and finally nobody bothering to look.
The tone is quiet and a little wistful, but not bleak. The ending turns on the idea that decomposing leaves feed the soil, so leaving the physical world also gives something back to it. The mood settles into a calm, thoughtful acceptance of change and endings.