In your question, you've used three rather different terms: "instructor", "supervisor", and "advisor". My understanding is that one generally doesn't have a specific faculty member as an adviser until one has advanced to candidacy (one may have a faculty member advising you, but one wouldn't have an official thesis adviser), and by the time one is a candidate, one should have mastered anything that is truly basic. That being said, while mastering the basics before advancing to candidacy is better than not mastering the basics before advancing to candidacy, not mastering the basics before advancing to candidacy and asking one's adviser for help is better than not mastering the basics before advancing to candidacy and not asking one's adviser for help. Furthermore, not knowing the basics before advancing to candidacy reflects as much on the school as you, as that is the whole point of qualifying exams.
As for an "instructor", it's their job to teach you everything in the course, and if there's material they expect you to come into the course knowing, it's their/the department's job to make that clear in the prerequisites. It's not your job to teach yourself the material; if you were going to do that, you wouldn't need an instructor in the first place.
If your friends are confused on this point as well, then that suggests it's not truly basic, or your program has failed to prepare its students on basic matters. Asking faculty members to explain helps not only you, but your fellow students (assuming you share what you find with them), and the faculty, as it hopefully will clue them in that students don't have a solid foundation in this area.
I'm afraid he'll think that I'm not willing to study the topic myself and that I'm wasting his time.
If you come to a faculty member, and say "These are the books that I've read on this subject, this is what I've learned from them, this is what I still don't understand, this is what my fellow students have said when I asked them for help, what do you have to help?", it would be difficult for them to conclude that you're not willing to put in work yourself.