If a person is insightful, they can understand what a situation or another person is really like, not just the surface. If a comment, a question, or a book is insightful, it shows that kind of understanding. It is a compliment, and you will hear it from teachers, in reviews, and at work. Though actually, people sometimes pin it on any smart remark; the tighter meaning is that the person has looked into the thing, not merely said something clever. It is not the same as being in sight — that only means you can see it with your eyes.