One can often see expressions like "Biology 101". Also in in book and article titles like "Wine tasting 101." What does this mean? Does this have something to do with a class one takes? Can it mean something outside of registering for a class?
It is a reference to the introductory lecture course on many topics (particularly those that are very large, e.g. chemistry, physics). It means that the course is a basic course designed to teach you the beginnings and some of the fundamental components of the topic.
This reference has been adopted into the English lexicon to mean the basic rules and practices which every participant in that activity is expected to know and follow: "Not sleeping with the Ambassador's daughter is Diplomacy 101," "Wearing a lab coat and glasses is Lab Safety 101."
As was pointed out below, the first number in 'topic 101' is often to indicate that the course is recommended to be taken in the first year of a degree, reflecting its introductory nature. By contrast 'ENGL 462' would typically be taken in later years. This nomenclature is not universal in this application, courses can be numbered, lettered or alphanumerically coded as well, and sometimes 'topic 101' is the basics of a very advanced topic, so not truly a basic course.
Edit: added contributions from comments