The speaker addresses an older woman, probably a grandmother or similar family figure, though the poem never says so outright. He begins with what he missed: her youth in Arkansas, when she was "young and trembling," bent over work like a sharecropper and making buttermilk for a "fierce husband." These few details suggest a hard, frightened early life, and the speaker can only imagine it.
Then the poem turns on the word "But." What he did see was her old age in the "squalor" of Northern California, when she was "full of Jesus." The speaker doesn't pretty up her circumstances. The poor surroundings are named plainly, and he loved her anyway, "deeply."
The mood is tender and quietly sad. There is respect for what she endured, and a sense that her faith carried her through. The trailing ellipsis at the end leaves the feeling open, as if the speaker is still thinking of her.