September rain falls on the house. (1)
The home is the center of the poem, and we learn about it in the very first line. And what's happening in that line tells us quite a bit about that home—it's got rain falling all over it (inside and out!).
In the failing light, the old grandmothersits in the kitchen with the child. (2-3)
The domestic scene is set, but it's tinged with darkness by the rain and failing light. Keep a weather eye out (sorry, we couldn't resist) for more places in the poem where the physical environment reflects the emotional life inside the home.
and the rain that beats on the roof of the house (8)
The home is under assault from outside forces. Or at least, that's how it feels in these lines. The rain seems violent and threatening here, and it's totally intruding on the grandma and child's cozy scene. Maybe it wasn't all that cozy to begin with, though.