Three short lines, and almost nothing happens: it is the end of August, and in the shade of a tree there is an old headstone. The poem simply points at what the speaker sees and leaves us to feel the rest.
The two images quietly echo each other. August's end is when summer starts to fade, and a headstone is where a life has already faded. Putting them side by side suggests that time is moving on, for the season and for people. The word "old" matters too. This grave has been here a long while, perhaps long enough that no one visits it anymore.
The speaker never says how they feel, but the mood seems calm and a little wistful. The tree's shadow could be read as cool shelter or as something darker, and the poem doesn't choose. It feels like a pause, a moment of noticing that everything passes, without panic or sadness spelled out.