Two key aspects of Plato's ethical doctrine are found in the Phaedo: the theory of recollection and the theory of forms. Although they differ they're both vital in supporting the other. Before understanding the theory of recollecting, I think a sound understanding of the theory of forms is necessary.
The theory of the forms is hinted at in previous books like the Meno but it isn't until the Phaedo before it is really introduced. The basic idea behind the theory of the forms is that in a sense, there are two realities. According to Plato, there is what we perceive to be "real and what is actually "real . The latter reality is what is actually real. This reality has always existed and will always continue to exist without changing. Nothing is tangible and there is no way to definitively prove their existence but Plato is more than confident they exist. Everything we typically believe to be reality isn't according to Plato. Understanding the forms is what is most important for Plato. Unless you know the form of something, you do not understand what is real. The idea of relativity also illustrates the importance of the forms. Using the example in the Euthyphro, this relationship is seen. Euthyphro is seeking to imprison his father for the murder of a servant. Some would argue that turning in a murderer regardless of his relationship to you is the just way to act. Others would say it is very unjust to testify against the one who is responsible for your being. Euthyphro's action could be considered both just and unjust. How can this be possible? Relatiivity is the answer. For the same reason that a pen sitting by itself can be considered both long and short, Euthyphro's actions can be just and unjust. Relativity must be conceded in a situation like this in order to understand what is really just. The only way to do that is to understand the form of jus