Plato's Parmenides is often said to be his most difficult dialogue. Consequently, my "reviewing" it here is likely to be of little help to any serious student of the work. Many far more qualified philosophers have tackled the work and come away with radically different ideas about its arguments and purposes. Nonetheless, I'll venture a short commentary, mostly for the personal benefit of recording my own understanding.
As difficult as it is (or perhaps because of its difficulty), the Parmenides offers the serious reader a transforming experience. Plato writes of a conversation mostly between Socrates and his older contemporary Parmenides. In most of Plato's dialogs, Socrates is the intellectual star, but here, Parmenides is the teacher. Broadly speaking, the Parmenides explores Plato's Theory of Forms and the metaphysical quandaries associated with them. Puzzling over arguments about unity, being, motion, time, likeness, and other fundamental metaphysical concepts pushes the reader to reflect on experience as one normally would not. Indeed, the similarity between the concentration that the Parmenides requires and meditation is striking.
Arnold Hermann's introductory essay provides a great deal of help for the novice. Arnold argues that the dialog is a demonstration that the forms cannot be considered in and of themselves, but that they are "interwoven" one with others and that they must be understood in the context of their application to the sensible world. If Arnold's understanding of the Parmenides is correct, it has a striking parallel in the Buddhist doctrine of the inter-relatedness of the objects of appearance. In the Parmenides Plato presents a series of apparently valid arguments for and against the forms having specific properties. This is similar to arguments in Buddhist scriptures for and against various ontological propositions. The conclusion that the Indian philosopher Nagarjuna draws from these arguments is that being is devoid of essence. All things of the world of appearance are impermanent or "empty." The manifold of the world arises only conditionally and in relation to all other things. Nagarjuna, however, goes a step further than Arnold to recognize a supreme truth or the emptiness (shunyata) that lies behind appearance, noting that it defies rational understanding and cannot be expressed in words, but only experienced directly by the enlightened. With Nagarjuna's conclusions in mind, it is not difficult to read the same ideas into the Parmenides, establishing two culturally independent formulations of the same metaphysical vision.
At least two other interpretations of the Parmenides also come to mind, however. First, it simply may be a critique of the Theory of Forms, with Parmenides taking Socrates through a series of reductio ad absurdum arguments. The simplicity of this interpretation is appealing; however, it seems unlikely in that early on in the dialog, Zeno and Parmenides tell Socrates that his explanation of the Theory of Forms is on the right track, but in need of refining.
If the Parmenides sought to provide a refined version of the Theory of the Forms, its positive contributions seem much less obvious than its critique. Pace Arnold, Plato may have intended to offer a refined version of the forms, but simply failed to complete this in the course of the dialog. That is, the Parmenides may be an unfinished work in which all that we have is the reductio ad absurdum. The constructive portion may have been lost in the course of time or Plato simply may have failed to produce the arguments he promised. This second interpretation would explain why the dialog is so difficult to understand.