The soul is born into the natural world from a pervious form, it forgets that knowledge. In this world, the soul has no experience of perfection, and therefore, cannot remember the forms. Yet, when the soul is confronted with something resembling the forms, it recollects what it once knew. When the soul resided in the invisible realm, it experienced perfect forms and retained that knowledge. This is called learning, but Plato believed it is actually recollection. For example, when we see two sticks that are the same length, we say that they are equal. Yet, there is nothing in the natural world that shows us true equality. The soul of an individual consist of three basic elements, first reason the master of all, spirit which is the rational part and then appetite the part that is out of balance or irrational. All humans possess the ability to realize Justice and Goodness if they can balance the three parts of the soul.
"Appetite" is the part of the soul which is lust, hunger, thirst and gets excided by other appetites. It is the irrational part of the soul that can rule all parts of the psyche when left unchecked by reason. The soul may be one thing, but it is on thing composed of multiple parts, one part that may desire what another part may resist, one concerned with objects that can be said to be in motion and motionless at the same time, and one concerned with the desire to drink (thirst) and the resistance to taking the drink one desires. If a soul can desire one thing (like alcoholics who may desire drinking alcohol), but will another (like alcoholics who resist taking a drink of alchol when that is what they desire) it seems to follow logically that these two moments suggest that one soul is composed of at least two parts because, according to the reasoning Socrates employs one thing cannot have opposing interest in itself in relation to the same thing.
"Spirit" is in the sense of a "spirited" horse.