Windfall is a tiny poem built around a single odd image: a piece of fruit that has dropped from a tree. The speaker looks at it and sees "a small heart / full of sickness." That's a startling comparison. The fruit has the shape and the ripeness of something alive, but inside it is spoiled or diseased, perhaps bruised or wormy from the fall.
The last line is the strangest. Calling the fruit "my sandwich's god" suggests the speaker is eating lunch beside it, or even that the fruit is somehow in charge of the meal. It's funny, but it's also uneasy. The ordinary food in the speaker's hand sits next to something decaying, and the poem seems to say that sweetness and rot live close together.
The mood is quiet, wry, and a little unsettling. The speaker doesn't panic or preach. He simply notices, and lets the odd, half-joking reverence hang in the air.