The speaker remembers childhood evenings when a mother working nights tucked them in with a strange kind of story: sad apes who lock themselves in boxes with no fruit. The speaker says this was the only subject worth telling a story about, which sounds both funny and a little bleak. It's a child's version of a grown-up idea, that people trap themselves and find little to feed them.
The second stanza moves outside. After the mother leaves, the crickets chirp and the stars seem to turn above a homemade slide the father built. These details feel warm and ordinary, a backyard made with care. Then the poem turns. Even as a small child, the speaker knew the moon shines because of a star that is burning itself out. The mood is quiet and tender, but with a shadow under it. The title fits: the child already senses that everything runs down, and the poem sets that awareness beside love and a familiar home, without making the two cancel each other out.