The speaker has been lying in bed for a long time, and the poem suggests a heavy, withdrawn kind of sadness. Grief here stops being a feeling and becomes an object: a stone. It is solid, cold, and hard to move, which fits the sense of someone who can't get up.
The second half widens into a strange image. The stone sits "in the book of prodigies," a phrase that points to a record of marvels and omens. That book was "ingeniously writ by water in a storm of hail." Hailstones are frozen water that falls hard, so the writing seems both clever and violent. It is also a bit contradictory, since water is a poor thing to write with and would melt or run.
The title, "Growth," adds irony. Maybe the sorrow is growing, like a stone gathering weight, or maybe the speaker is slowly changing through it. The mood is quiet, heavy, and oddly wondering, as if the speaker sees their pain as something both strange and remarkable.