The topline goes from the poll (i.e behind horses ears) all the way along the neck and back to the tail. If the horse has a hoid topline it means it has well developed muscles im the neck and the back and looks fit and round rather than hollow and bony. Hope that helps!
The topline is essentially the muscle groups along the horses spine. A topline of a horse goes from the wither area to the croup.
Also, no question is stupid! It’s okay to ask questions, even if you think it’s something that you should know but you don’t. I can guarantee that other people on Horsealot will see this question and think “I’m so glad someone asked! I was too nervous.”
I hope this answers your question!