As a prelude to Plato's Phaedo, the Meno introduces the theory that knowledge comes by recollection and is further examined in the former work.  Nonetheless, Socrates introduces this idea and it deserves some clarification and explanation.  Plato believed that the soul was immortal.  Being such, it also had knowledge that was simply forgotten by the body during birth.  And to learn one must remember, through recollection prompted by the right questions, that which it has forgotten but does indeed know.  This is what Socrates explains to Meno in the famous slave/geometry illustration of recollection.  By this method, Meno's paradox is refuted and learning is possible. 

Another important idea that comes from this work is Meno's Paradox, or as previously called, the Paradox of Inquiry.  The argument is formulated as follows:

Assumption: Suppose one wants to come to know some fact P.
Premise 1: One must inquire either knowing or not knowing.
Premise 2: If one knows, inquiry is unnecessary.
Premise 3: If one does not know, inquiry is impossible.
Conclusion: Inquiry is unnecessary or impossible.

This proof is called a paradox because it is a good argument that yields a false conclusion, namely that one cannot move from ignorance to knowledge.

Plato's (and Socrates') answer to this is that you can move from one to the other by recollection.  You must make conscious what has been latent, and then learning is possible.  Some questions that may be considered with regard to Meno's Paradox are whether or not the soul goes from not knowing to knowing, where one ends up after recollection in terms of belief and knowledge, and what are the various states involved other than remembrance? Other philosophers have considered these questions and some are answered in Plato's Phaedo, but they still remain up for discussion, which is the beauty of Plato and the issues he raises in his dialogues.

In the beginning of the text, the essence criterion is implicated by Socrates when he asks Meno for a definiton of virtue.  In all Socratic dialogues it is necessary and demanded by Socrates that the true nature of that which one seeks to understand is defined by its essence.  Examples and fragmented definitions are not sufficient to define anything according to Socrates, and he always requires that there is a definitional knowledge that reveals the true nature of the thing before any further inquiry is taken on.  His first priority is to correctly define some thing, and from there he pursues on the path to wisdom-in this case the thing is virtue, and he tries to figure out how one acquires virtue (if it can be taught or not as asked by Meno).

Also in this dialogue is the mention of it being psychologically impossible for people to desire bad things.  This is important because it causes a lot of debate between philosophers and the lay readers as well.  Socrates relies on this belief in part during his trial when answering accusations of corrupting Athens' youth. 

Finally the qualities and differences between true opinion and knowledge are discussed near the end of the Meno.  True opinion can in some cases be as good as knowledge because they can both yield the correct answer or truth.  The latter, however, is stable in a way that true opinion is not because of an account of reason that one needs to have knowledge.  The reason that one has for possessing knowledge is recollection and because of this stability, knowledge is more valuable than true opinion-it will always provide the correct answer and truths to questions.