The speaker looks back three years and describes a time when they were "stone": calm, painless, and wanting nothing. The title, "Butsuzo," is a Japanese word for a Buddha image, so the poem seems to compare the speaker to a statue sitting in stillness. That reading fits the last lines, where all that remains is light, a single room, and the act of sitting.
What makes the poem interesting is how unsettled its calm feels. Being free from pain sounds like a relief, but the speaker also had "no joy or urge." Nothing hurts, and nothing is wanted either. The plain, short lines mirror that emptiness, and the quiet final line, "and I sat," lands like a full stop.
The mood is peaceful but a little distant. Because the speaker says "was" stone, the poem suggests that this state has ended. We're left wondering whether they miss that stillness or are glad to have left it behind.