The Apology is a text by Plato in which he tells us how Socrates defended himself against those who accused him of corrupting the youth and not recognizing the gods of the state. Because of these charges, Socrates was put on trial.
The confusion about the nature of the work comes from its title. The work is called The Apology. However, Socrates is not apologizing, in the way we understand this in modern times. Apologia translates to something like "defense," and this is what Socrates does. In this work, he defends himself against his accusers.