The eye, Plato says, is unusual among the sense organs in that it needs a medium, namely light, in order to operate. The strongest and best source of light is the sun; with it, we can discern objects clearly. Analogous things, he writes, can be said of intelligible objects (i.e., the fixed and eternal forms that are the ultimate objects of scientific and philosophical study):
By "the world of becoming and passing away" Plato means the familiar visual or perceptual world we see around us. Thus if we attempt to understand why things are as they are, and what general categories can be used to understand various particulars around us, without reference to any forms (universals[?]), we will fail completely, as if [we] lacked reason. By contrast, "the domain where truth and reality shine resplendent" is none other than Plato's world of forms--illuminated by the highest of the forms, that of the Good. Since true being resides in the world of the forms, we must direct our intellects there to have knowledge, on Plato's view; otherwise, we are stuck with mere opinion of what may be likened to passing shadows.
Plato also says the sun and the Good ("the object of knowledge") are both sources of "generation":
This is one of the passages that leads some to infer that the Good is, for Plato, God, though there is some dispute about this point. Many modern readers will find it puzzling that one and the same thing is called the Good, the source of being (the being of the forms, at least), something that (somehow) sheds light on all other forms, and a universal. Indeed, exactly how it is Plato thinks "very existence and essence is derived to [the forms] from" the Good is a matter of considerable interpretive difficulty.
This doctrine conveyed by the metaphor of the sun is, incidentally, an excellent example of how, traditionally, the subjects of metaphysics and epistemology have been closely intertwined: accounts of what exists, at a fundamental level, often deeply inform (and are informed by) accounts of ways or kinds of knowing. It also neatly sums up two views for which Plato is well-known: his rationalism and his realism (about universals).
Plato goes on to describe the levels of reality and knowledge with the device of the so-called "divided line" (509d-513e). Immediately afterwards, at the beginning of Book VII, the same doctrine is elaborated using the famous allegory of the cave (514a-520a).