When you are learning a new subject or even extending your knowledge of an old subject, it is terribly easy to get confused or lost. That's why schools were invented! To have other people there to help you through those confusing times.
Generally your first place to look for an answer is your lecture notes. If your teacher provides you with copies of lecture notes or slides, use those instead of the ones you wrote -- they'll almost invariably be better (just because the teacher wasn't trying to keep up with herself, not because you're dumb...touchy, aren't we? *grin*). If you've found the answer, GREAT!
If not, your next source would be any notes your teacher may have given you as a 'supplement' to the lecture notes and/or book. These often have bits that the teacher knows have confounded students in the past and deserved special attention. If your answer is there, WONDERFUL!
If not, check next in your book(s) and the notes you've taken on the book(s). (Sick, eh? Taking notes about a book. But it can help if for no other reason than sheer repetition of the information.)
While looking through the book, check the section(s) you think this topic was in first. After that, try the index (in the back). If you can't think of the right word for that, try the table of contents. If still baffled, maybe you should have reconsidered taking those notes on the book. (*grin*)
Finally, when all your resources have failed you, ask your instructor. Office hours are present so that teachers can answer one-on-one questions. Sometimes it is something you couldn't find like the scenario above. Sometimes it is something slightly off topic you'd like information or pointers on. Sometimes it is something you didn't want to ask during class for whatever reason. Feel free to call or go by the professor's office during their office hours.
If those aren't convenient, most teachers will make appointments. Most schools have voice mail and snail mail boxes for their instructors. And, in this technological age, almost everyone (teacher or not) has email. (If you don't have one, see if the school will provide you with a free email account. Most do.)
(Point of etiquette: ask the person you'll want to contact for their preferred email. Many people have multiple email accounts. Some separate work from friends with a separate email address. Although most businesses give employees email, the person may have a preferred alternative. I know I hate the email app that my school provides...Eudora...*shiver*)