The speaker begins by sitting with three heavy feelings: duty, guilt, and shame. Calling himself "a brother to the poor" suggests he feels close to people who have little, or perhaps that he is uneasy about how much more he has. The first two lines are weighed down, almost like a confession.
Then the last line shifts. On "this day" he starts putting buckles on his shoes, a small, ordinary act that sounds oddly significant. Buckles might suggest some polish, fastening, or readiness, though the poem doesn't spell out what they mean. One possible reading is that he is finally moving from brooding to doing something, however modest. Another is that the buckles are a bit of finery that sits uncomfortably beside his talk of poverty. The poem leaves that tension open.
The mood is quiet, uneasy, and a little ironic. It's short and plain, and the plainness makes the speaker's mix of self-reproach and small change feel honest.