The metaphysical role of the Forms is to distinguish the truly real from the merely appearing. Physical objects present appearances of the forms, but they are never "perfect" and fall short of the "ideal" set by the Form. A drawing of a circle, for example, no matter how carefully constructed, is never the geometer's perfect circle, the object of geometrical knowledge. What is physical and can be grasped with the senses is always changing, coming into being and perishing. It thus lies "between" what truly is, the Form, and "that which is not," the object of complete ignorance. So just as "opinions" can be said to lie "between" knowledge and ignorance, the objects of opinion lie between Being and Non-being, they are said to be in a state of "Becoming" for they are always becoming something else, they are never purely and simply "what is."
Thus the theory of Forms gives Plato a "two-level" metaphysics
or theory of the nature of reality.
One level is called the "Realm of Being" containing the Forms. These are the true realities, the objects of knowledge. They neither change nor come into being nor perish; they are eternal. Each one is absolutely unique and different from the others; there is only one Form of the Just, one of the Beautiful. etc. If you and I differ about what is "just" we cannot both have knowledge, one or the other or possibly both of us have mistaken opinions. The unity of each Form guarantees that those who have knowledge will always agree, for they "see" with their minds the same one reality. Furthermore, the fact that knowledge takes as its objects the Forms and that the Forms are changeless and eternal guarantees that knowledge is "certain" or "necessarily true" in the sense that it if we have genuine knowledge, what we know to be true today cannot be false tomorrow. By contrast, opinions which take as their objects the physical objects we perceive with our senses can be true at one time and false at another, for such objects change. Opinions are therefore capable of being true at one time and false at another; they are said to be "fallible" whereas knowledge is "certain."
Physical objects, seen by the bodily senses, form the second level of Plato theory of reality; they exist in the "Realm of Becoming." This is the physical world, the world of space and time in which one's body dwells. Such objects come into being and perish, they are constantly "in flux", changing. What is true of a physical object today may be false tomorrow; thus our state of mind regarding physical objects must always be restricted to fallible opinions.
However, physical objects are not totally un-real, for they really are, at least for the moment, this or that to some extent or another. They have this degree of reality because physical objects bear a relationship to the Forms; Plato says they "share in" or "participate in" or "reflect" or "copy" or "imitate" the Forms. (The nature of this relationship is always somewhat elusive, and in his elder years Plato himself directed stinging criticisms against the theory on this grounds.) The basic idea is that something is what it is to the extent that it shares in the Form of that thing; to the extent that it fails to do so, it fails to be that thing. A man is "just" to the extent that his life copies the Form of Justice Itself, and "unjust" to the extent that it fails to do so. Thus the metaphysical role of the Forms is that they make things what they are.