101's stereotypically the introduction to the course, but this sort of thing actually varies quite a bit between universities. Mine dropped the first digit for survey courses and introductory material; survey courses were generally higher two-digit numbers (i.e. Geology 64, Planetary Geology), while introductory courses were more often one-digit or lower two-digit numbers (i.e. Math 3A, Introduction to Calculus). Courses intended to be taken in sequence had a letter appended. Aside from survey courses, higher numbers generally indicated more advanced or specialized classes, though not necessarily more difficult ones.

Three digits indicated an upper-division (i.e. nominally junior- or senior-level) or graduate-level course. Upper-division undergrad courses were usually 100-level, and the 101 course was usually the first class you'd take that was intended only for people of your major; CS 101 was Algorithms and Abstract Data Types for me, for example, and I took it late in my sophomore year. Graduate courses were 200-level or higher.