What does "upper division" mean in the context of undergraduate classes?
Exactly as you said. In the U.S., undergraduate courses are commonly sorted by level, or the year of study in which they are likely to be taken. See this pattern in the traditional course numbering scheme of 1-99 for remedial courses, 101-199 for first year courses, 201-299 for those of the second year, etc. In this context, "upper division" are those courses for upperclassmen, those past the midpoint of their studies: the ones numbered 301-499.