What do they mean by challenging classes?

I’m thinking of transferring into elite/semi-elite schools.
I have a very high, near-perfect GPA. However, my first year I took mostly introductory courses to my major and general education courses. I took 15 credits each semester. I haven’t taken any high level courses yet – didn’t think they were necessary, and many of them have pre-reqs so I can’t take them as electives if they’re not in my major. Can you give me some examples of “challenging” courses a freshman/sophomore could take?

Thank you!

you need to tell us your major because “challenging” courses varies for everyone.