If Plato's dialogues in general are notable for their depth within a relatively straightforward framework, the Meno is particularly so. At first glance, the dialogue seems to proceed quite clearly (albeit with a few somewhat involuted sections, such as the geometrical quiz given to Meno's slave). It also seems to settle or establish very little--in the end, no definitive answer is given to the text's central question of what virtue is.
This simplicity and inconclusiveness, however, hide an extremely ambitious set of goals. The first such project we encounter concerns the nature of a definition, a concept quite new in Socrates' time and largely at odds with the received wisdom of ordinary Greek citizens. That the nature of virtue could even be a question is remarkable to Meno (and presumably to Plato's early readers)--indeed, he opens the dialogue not by asking what virtue is, but rather if and how virtue can be taught.
Thus, much of the initial dialogue is devoted to the idea that virtue must be rigorously defined before we can deal with subsequent questions about it. This point is at the heart of the Socratic elenchus, which seeks to clear the ground of received, unconsidered knowledge in favor of the pursuit of truth. Meno confidently offers a number of definitions of virtue, but each of them merely cobbles together various aspects of Greek cultural custom. Socrates then dissects these to show that they do not meet the requirements of a definition. Thus, on the pretense of determining what virtue is, Socrates actually pursues the prior project of showing what fundamental virtue is not. What is really accomplished in the Meno is not a theory about virtue but rather a theory about what is necessary to frame a good theory about virtue.
The first such necessity is attention to what is truly universal about "virtue." Meno's most common error involves naming various examples of virtue instead of naming what is common to all the examples. A closely related necessity for a definition is that it cannot use the term to be defined within the definition itself. Socrates makes this point in the context of Meno's idea that virtue is the ability to acquire beautiful things. Socrates makes Meno admit that such acquisition is virtuous only if it is just. But if justice is a virtue, it cannot be used in the definition of virtue (i.e., Meno has basically defined virtue as the acquisition of beautiful things in the context of a type of virtue).
This is truly an awesome project--Socrates (and Plato after him) is trying to convince a world that has always been confident in its knowledge that it in fact knows nothing about the things of which it is most certain. What is even more striking is that he is trying to convince the world not only that it does not know, but also that it does not even know how to know. Socrates makes no claim to know the real answer to the question of virtue, but he does claim to know the basic form that such an answer would take.
Nonetheless, this radical destabilization of everybody's most heartfelt knowledge about goodness is a painful and disorienting process for Socrates' interlocutors, who are repeatedly flabbergasted by what they now seem not to know. This uncertainty comes to a head in the paradox about seeking what one does not know, which Meno brings up after one of Socrates' unforgiving deconstructions. How are we to look for virtue without first knowing what it looks like?
This question inspires Socrates to introduce an early version of his idea of anamnesis--the idea that learning truth is really a matter of the soul recollecting what it has learned before its current human birth. This idea has always been a major focal point for readers of Plato, partly because it seems to be a radical departure from Socrates' constant claims that he knows he knows nothing. The theory of anamnesis seems to be a glaringly positive piece of theory amongst a heap of negatives and deconstructions.
In the end, Socrates has in fact made a few substantive points about virtue besides the point that to learn it (if it were knowledge) would actually be to recall it. The most important such point is that the good or virtuous depends on wisdom: "All that the soul undertakes and endures, if directed by wisdom, ends in happiness." This will be a recurring theme in the rest of Plato's work--true virtue is not a matter of custom, but rather of knowledge.
In the Meno, however, this is not stated clearly. There is a lingering conflict between the conclusion that virtue is, "as a whole or in part," a kind of wisdom and the conclusion that no one can teach it (so that it cannot be knowledge). The Meno leaves us hanging between defining virtue as straight knowledge or as a kind of mysterious wisdom revealed to us by the gods "without understanding." It is seen as likely that most virtuous men are so by holding "right opinions" rather than true knowledge. Right opinions lead us to the same ends as knowledge, but do not stay with us because they are not "tied down" by an account of why they are right. Thus, we can only depend on semi-divine inspiration to keep us focused on right opinions rather than wrong ones.
This dilemma brings us back to Socrates' (and Plato's) original purpose--the mode of dialogic analysis Socrates pursues with Meno is meant first of all to show up wrong opinions. Secondly, it is meant to clear the ground for an inversion of the whole sequence of right opinion and truth. If the requirements for a definition of virtue can be filled, we would no longer need to test out opinions blindly (as is done throughout the Meno). Rather, we would have an account of virtue first--an idea of virtue that is "tied down"--and could determine the details from there. The Meno only pursues the first part of this project, but it lays a great deal of groundwork for the second.