Thought it'd be more fun and interesting if I was vague:-)

As in US college courses i.e. XXXXXXX 101. I presume it means (from colloquial usage I've heard) a beginner's class but what does it acually mean? Something like the first lesson (module) in the first year of study which would be followed by XXXXXXX 102, sophomore year (2nd year to me) classes would be 201 etc?