This comes from typical university course notation. Though the exact method varies, higher numbers indicate harder courses, with the first digit typically used as a sort of level grouping (e.g. 100s=Intro courses, 200s=more specific topics, 300s=only intended for majors). It provides a standardized course number in addition to the course title. "101" will thus be the most general introduction to a subject, often a pre-req for later classes. So when we say "History 101" or "Cooking 101" or "Reddit Comments 101" we're treating it as if it was the basic intro course in that subject at a university.