I should add that they are more expensive because they are usually larger and the components are made from sturdier materials. But "professional level kit" isn't a phrase or a concept that is used in hobby rocketry; it is salesman-speech. I think the web store is just trying to say that these kits aren't for rank beginners, and, as I mentioned before, to provide some shorthand way of saying why they are more expensive than the other kits there. If you are buying online, it can be hard to see just from the little pictures that are attached to each product listing why those rocket kits carry much higher price tags than the kits from, say, Estes or Quest. The "professional level" label gives you sort a clue that they are indeed different from the other kits.