The title is a Latin phrase meaning "with a grain of salt," and the poem plays on it. The speaker says that "for my sins" they ate only a little salt, as if a bit of kitchen seasoning were a small religious penance. The tone is wry, a little self-mocking, as though the speaker is shrugging at their own failings.
Then the poem turns outward. The first hard frost arrives, and it becomes "a cold supper." Frost looks like salt scattered over the ground, so the speaker's meagre meal and the frozen morning seem to blend together. The season itself is the dish being served, and it is thin and chilly.
The mood is quiet, sparse, and slightly humorous. There is a hint of rueful acceptance: the speaker takes the cold as a kind of consequence, and doesn't complain much. The poem leaves us with the feeling of autumn tipping into winter, and a person making do with very little.