Socratic irony is when you pretend to be ignorant to expose the ignorance or inconsistency of someone else.
Socrates was a famous Greek philosopher known for his probing questions. Irony is when a statement means the opposite of its literal meaning. That should help you remember that Socratic irony is a technique where the questioner admits (falsely) to not knowing something as a way of tricking the other person into revealing his own lack of knowledge or a flaw in his logic. Socratic irony involves pretending to be ignorant to show someone else is ignorant: thus, the irony.