Course numbers indicate the difficulty level of the course.
Generally, 100 series courses are considered freshmen-level, 200 series are sophomore, 300 junior, and 400 senior-level.
Often higher-level classes will have other classes as prerequisites.
Don't take 300 or 400-level courses your first year. After you've succeeded in getting good grades at lower-level courses, then take higher-level classes. Most 300 or 400-level courses have prerequisites, but not all. Make sure you understand the difficulty level before signing up for a class!