Three short lines, and almost nothing happens, which is the point. Rain arrives in the morning, and the speaker is still in bed, listening. The poem seems to treat that sound as a kind of goodbye, though it never says who or what is being left behind. That gap is where the feeling lives.
The title, "Seasoning," gives a clue. It suggests time passing, a change from one season to another, or the slow way experience flavors a life. Judging by the imagery, the farewell could be to summer, to a person, or to a stage of life. The poem leaves it open, so readers can fill it with their own loss.
The mood is hushed and tender, a little sad but not dramatic. The speaker doesn't rush or protest. Lying in bed while the rain comes in suggests acceptance, even comfort. The line breaks add to this, since each phrase sits alone, like a pause between raindrops.