This is a system that works using pairwise comparisons and ELO. In the background every vote affects the overall ELO of the course using the Glicko2 algorithm. You can read more about it here. With every vote, we become a little more sure of the subjective difficulty of these courses. An ELO system is better than a simple rating since individual's scales can drastically differ.
If you can't compare the two classes for whatever reason just refresh the page it will spit out 2 new ones.