The Apology is Plato's account of the speech given by Socrates after he was accused by the state of both corrupting the youth and of believing in the wrong gods. There is no real way to know if this is a factual account, but if it is, the Apology is an invaluable historical document detailing the last statements of an immortal philosopher. Even if it's not so accurate, this address is a fundamental defense of free speech and individualism, rights for which Socrates lived and ultimately died. This work also delves further into previous themes such as interpersonal relationships, the nature of goodness, and issues of life and death.
The Apology opens up with Socrates stating that his peers, the men of Athens, haven't been persuaded by the accusers. Socrates then states that philosophy begins with an admission of ignorance; thus whatever wisdom he has stems from the knowledge that he knows nothing.