"101" is used to mean the basic or survey course of a field of study in college. Not all colleges use this exact form (some would call this 1001, some 111, and so on), but it is fairly standard practice to have an introductory course labeled with this number.
From this meaning of "basic information", it has obtained a slang meaning of "simple information" or even "over-simplified information". For example, "Don't talk about yourself too much on a first date" could be referred to as "Dating 101". "Learn about the neighborhood the house is in" could be referred to as "Real Estate 101", and so on.