The Socratic method
Although there is a famous pedagogy by this name where a teacher will let students "teach themselves" by asking wise questions, that is simply not the version that Plato depicts. Plato's Socrates has specific beliefs that he is teaching, but he is doing that in a way that the students can participate in so they feel autonomous. There is a symbol built into this model, obviously, because these are really Plato's arguments after all, so Socrates and Theaetetus are literally symbols for Plato and the reader.
The trial as a symbol
Because the nature of this writing is that it is historical but also imaginative, the trial is therefore a symbol specifying an irony that lies between Theaetetus and Socrates. Theaetetus believes that the questions are designed to make him feel better—after all, he was given over to Socrates as a kind of gift to him by his first mentor, Theodorus. The fact that Socrates is on trial the whole time is a symbolic reminder that what makes Socrates enlightened is that his questions are a response to his awareness of his impending death.
Theodorus
As a teacher, we don't learn much about Theodorus, except that he is Theaetetus's first teacher. He is also a symbolic character because he represents Theaetetus's qualification to sit at Socrates's feet and be his student; it is that he has already proven himself to be a competent student for his minor teacher. Theodorus represents Theaetetus's reason for studying in the first place: Curiosity. Also his name has theological implications.
The first mistake
We learn something symbolically from Theaetetus because of his first mistake with Socrates. Socrates asks him to define knowledge, but instead, Theaetetus explains the utility of knowledge, showing that in Plato's opinion, the truth is often hidden in plain sight, not because true knowledge is impossible, but because humans are designed to feel like everything automatically makes sense, when the true nature of reality is unfathomable and ordered, seemingly by design, as their "Logos" conversations indicate.
Theaetetus as a symbol for the reader
Throughout the book, the reader might get the opinion that perhaps Socrates is talking not to Theaetetus but to the reader. That is because Plato designed Socrates character to exactly that effect. Plato has interests beyond just telling a nice story about an old friend of his. He wants to exploit Socrates as a symbol, and therefore, Theaetetus is also a symbol. This motif shows that Plato feels the reader can still gain the enlightenment he is discussing.
