system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: WTF is the number after classes in the US? Informatics 101, etc.
gpt
100 and 200 level class are general education and introductary classes to a subject.  Usually they can be taken by anyone regardless of major.

300 and 400 level classes are more advanced classes and are generally only for the people who are involved in a major for those classes.

All 100 to 400 level classes are for undergrads

500 and 600 level classes are for graduate students.

700 and above are usually for doctoral candidates and some grad students.

The number after the hundreds, such as 101, or 102. Just designates a specific number for the class, and many schools treat it differently. Usually a 101 class is the first class (or basic class) in any subject.

Sometimes classes are in a sequence as well, such as 351 and 352. As  in it may say MATH 351 Algebra, then MATH 352 Algebra 2.