Ask five Graded teachers and/or students what Theory of Knowledge (ToK) is about and you'll get five different answers. Chances are that many of the answers will be totally off base. It's one of the classes that you have to experience to truly understand what it means. Of course, even after 3 semesters students leave the course with more questions than answers. According to the guide, the most central question is "How do we know?"
As of this year, the IBO opened up ToK to all students, even if you're not an IB diploma candidate. If you're looking for a taste of what this year's juniors have done, check out their "This I Believe" audio statements.