A poor box is the slot in a church or shrine where people drop spare coins for those in need. Here, the poem imagines what ends up inside it, and the answer is strange and unsettling: not money, but "one heart," "one people's hunger," and "one time to be old / in a silk bed."
The speaker never says outright how they feel, but the tone is quiet and a little sorrowful. The things in the box are the things the poor have to give up or go without: love, enough food, and the comfort of a gentle old age. By turning these into offerings, the poem suggests that the poor are asked to give what can't really be spared. The silk bed stands for a comfortable end to life, which many people never get.
The mood is spare and sombre. With so few words and no decoration, the poem feels like a small, hard fact set down plainly, leaving the reader to feel its weight.