What is Plotinus saying in the eighth tractate about the souls decent into body?

Just to be clear, this matter is discussed by Plotinus in the fourth Ennead, suitably entitled "On the soul's descent into the body." Heavily influenced by Plato, Plotinus sees the soul as essentially trapped inside the body. As a spiritual entity, it has more reality for Plotinus than the merely physical body. This particular section of the book is important because it illustrates Plotinus's general system of metaphysics, which is based on a hierarchy of being.
At the top of this ontological hierarchy (ontological means relating to the nature of being) sits the perfect, indivisible One. All lesser forms of being such as intellect, bodies, plants, animals, and minerals are further down, ultimately emanating from the One. A good way of understanding Plotinus's philosophical system is to try to see it as a kind of pyramid with the One at the very top and various types of matter, such as bodies, right at the bottom.
For Plotinus, the soul is superior to the body because it partakes of the eternal. The body, on the other hand, is restricted to this world; it is susceptible to decay and corruption, and it will eventually die. Plotinus describes the body as "the house of poverty" for the soul. Yet, the fact that the soul is housed in the body does not necessarily have to be a bad thing. The soul is unique in that it can look two ways, as it were. Although it has descended into the body, it can still focus on the higher realities; it can still partake of the eternal. All too often, however, the body disturbs the soul, attacking it with hunger, pleasures, desires, sorrows, and all other kinds of distraction. These divert the soul's attention from reconnecting to the world of higher things.
The soul is caught between the higher and lower stages of Plotinus's hierarchy of being; the soul has something from above and something from below. It was by focusing on the things of this world, the lower stage of reality, that the soul originally became individualized and first came to reside within the body. However, if the soul chooses to turn its attention to higher things, to the eternal truths, then it can no longer be damaged by its temporary home in an imperfect body. It can ascend, by way of a mystical experience, back to where it came from: the One, from which the soul, as with every other being, originated.