The Nature of The Afterlife
Although this poem is often considered to be a "death poem", in actuality it is more of a "life after death" poem; the narrator, speaking to a loved one still living, is trying to explain the nature of death and what comes after. She is not going to be lying in a box under the ground, but rather, her soul will be part of the next life and also become a part of the natural world. The poem is putting forward the suggestion that death is not the end, it is merely a changing of state and that there our other realms of our existence that the living can still connect with.
Grief
Another of the poem's themes is grief; the narrator knows that the loved one she has left is grieving deeply for her, and in some ways is trying to change the nature of her grief. Instead of missing her, she wants her loved one to look for signs of her, knowing that this will make the grieving process easier.