Let me say this. First, put Phaedo in context. This could be encapsulated in the overall question "what is the meaning of death?" Socrates is explaining (for our purposes) his position on the question of an afterlife. Socrates thinks, after much translation and rereading of the text, that the physical body is a hindrance, that it is something we have to live with while we exist in the material world. Thus the real world is "immaterial" to Socrates. Our life if lived well moves us closer to reality. But our embodied life means nothing.
Cebes' objection is, simply put, that he refutes Socrates concept of a transmigrating soul (think Buddhists and their conception of the soul and reincarnation) that is, that the soul (the republic
conception of it) keeps all of its memories and knowledge enroute.
This what I remember at any rate.
What class is this for, because the class can sometimes give you a hint as to what aspect you should evaluate a question under.