101 is a topic for beginners in any area. It has all the basic principles and concepts that is expected in a particular field.
In American university course numbering systems, the number 101 is often used for an introductory course at a beginner's level in a department's subject area. This common numbering system was designed to make transfer between colleges easier. In theory, any numbered course in one academic institution should bring a student to the same standard as a similarly numbered course at other institutions.
History
The term was first introduced by the University of Buffalo in 1929. It was used as a course catalog, the first known usage of the term by Oxford English dictionary.
Based on this usage, the term "101" has been extended to mean an introductory level of learning or a collection of introductory materials to a topic.