There is not a great deal of context that is crucial to understanding the essential themes of the Meno, largely because the dialogue sits nearly at the beginning of western philosophy. Socrates and Plato are working not so much in the context of previous philosophies as in the context of the lack of them. Further, this is very probably one of Plato's earliest surviving dialogues, set in about 402 BCE (by extension, we might presume that it represents Socrates at a relatively early stage in his own thought). Nonetheless, in order to understand the aims and achievements of the dialogue, it helps to keep in mind some details about this lack of previous philosophies.
Since neither virtue nor any other concept has yet been defined in the way to which we are now accustomed, Socrates has to show that defining these things at all is a good idea. In this task, his primary foe is Greek cultural custom and the political aristocracy that most strongly embodies that custom. Meno, a prominent Thessalian who is visiting Athens, is a member of this class. Meno's semi-foreign status aids Socrates (and Plato) in the dialogue, allowing for eyewitness accounts that Socrates himself could not give. Thus, Meno is able to say with authority that the Thessalians do not have anyone who can clearly teach virtue, while Socrates (and Anytus, a prominent Athenian statesman) can vouch for the sorry state of affairs in Athens.
Meno is also a handy interlocutor for this dialogue because he is a follower of Gorgias, one of the most reputable of the Sophist teachers, and knows the Thessalian Sophist community to some extent. He therefore serves as a Sophist foil for Socrates' logical points. This is not quite a fair fight, of course, since Plato can put wh ...