The Meno is probably one of Plato's earliest dialogues, with the conversation dateable to about 402 BCE. The dialogue begins with Meno asking Socrates whether virtue can be taught, and this question (along with the more fundamental question of what virtue is) occupies the two men for the entirety of the text.
Important and recurring Platonic themes are introduced in the Meno, including the form of the Socratic dialogue itself. Socrates attempts to dissect an ethical term by questioning a person who claims to know the term's meaning, and eventually concludes that neither he nor the "expert" really know what the term means. Other important themes raised here in an early form include that of anamnesis (the idea that the soul is eternal, knows everything, and only has to "recollect" in order to learn) and that of virtue as a kind of wisdom. Socrates also makes a number of essential points about the nature of a definition.
Socrates and Meno work through a number of possible definitions of virtue, each suggested by Meno and dismantled by Socrates. At one point, the question is raised whether it is even possible to seek for something one does not yet know (as in the case of seeking a definition of virtue), and Socrates performs a scale-model elenchus with Meno's slave to solve the problem via the theory of anamnesis.
By the end of the dialogue, the participants (which include Anytus, who enters toward the end and has a minor role) have arrived at the classic state of Socratic aporia--they still do not know what virtue is, but at least they now know that they do not know.