The speaker imagines their own death and asks to be buried so that a tree can grow from the body. It's a calm, practical request: seeds, a heavy layer of dirt, and then new life rising from "decaying mortality." The tree is a modest hope ("if nothing else"), a living reminder of the speaker and of the bond between speaker and reader.
Midway through, the poem widens. The speaker points to a "source" that flows in every direction, "left, right, above, and below," in ways we can't fully understand. Death stops looking like an ending and becomes one part of a larger cycle that connects everyone.
The last line turns the focus to the living: "If we are breathing, we've still got time to grow." The mood is quiet, tender, and hopeful rather than grim. The speaker isn't afraid of dying, and uses that calm to nudge the reader to make use of the time they still have.