From what I've heard, in high school you can take either the normal class or an honors class, which is more challenging, I think. You'd take an honors class if you are really good at a subject- for example, my art teacher asked me if I was going to take Art 10 honors (Which I think I will).
There are no honors classes for grade 9, so I'm assuming they're for grade 10 and above. (In my school, at least)
You probably didn't know what it was because you're in middle school, though I am unsure if Sweden has honors classes in high school in the first place.