A roof rack is like a special shelf that you can put on top of a car. It can be used to carry things that are too big to fit inside the car, like bikes, canoes, or luggage. The roof rack has special bars that go across the top of the car and hold the things you want to bring along. You can attach things to the bars using special straps or clamps. The roof rack is really helpful when you need to bring lots of things with you on a car trip. But, you need to be careful when putting things on the roof rack so they stay secure and don't fall off while you're driving.