Imagery is the use of words to create a mental picture--which Lucille Clifton was very good at doing.
This poem is about growing old in body but not in spirit. Clifton says there is a young girl inside her who "will not walk away/ and leave these bones/ to an old woman." She will not let the condition of her body make her mind and spirit old.
Clifton uses the image of a green tree in a "forest of kindling," meaning dried-out wood ready for burning, to symbolize the young spirit, the "green girl," inside her.
She uses the image of a nun waiting patiently for the second coming to symbolize her patience in waiting for that day, presumably after death, when she will have the new body promised to believers (I realize I'm reading a lot into this). She returns to the image of the green tree to symbolize that new, young body.
She turns from religious imagery to recall the metaphor of being "randy as a wolf" (meaning she's in "heat") from the first stanza. The wild growth of the woods after her lovers have "harvested" her "honey and thyme" symbolizes the sexual potency and fertility of her new body.