This phrase is an example of so-called Socratic irony. As a reader of
Plato's work, you will discover that, despite
Socrates' claim of ignorance, he is no dummy!
This often leads a student to believe that
Socrates is being dishonest and is merely laying
intellectual traps for those with whom he converses. Try to think of it this
way. If knowledge is defined as revisable claims that can be defended, then
Socrates is being deceptive because he
certainly has an understanding of such things. If, however, true knowledge
is final and beyond revision, Socrates is being
honest. He doesn't see himself as having reached that level of understanding.
In either event, his professed ignorance challenges various self-proclaimed
experts in the dialogue to join the process of discussion.