He means "... you needn't know."

A classical education is more about technique than material or school. The standard education model is that students are told what is true and what is not, and then they need to regurgitate what they've been told. The classical approach is for a student to try to understand the material themselves, then interact with a teacher and fellow classmates through a discussion format based on the Socratic method to develop their understanding.

Here is an example of this technique at work with elementary school children, teaching them binary numbers: http://www.garlikov.com/Soc_Meth.html