Pay sites usually refer to something paid. The main meaning is that you are going to pay for some fun which is only dedicated to the eyes of adults. That means you need to be more 18 years old. But there are also countries where you need to achieve 21.
Pay sites is the opposite of the free websites. But not only because they are paid. They often offer exclusive and premium content, which you can not find on common freemium websites. Next factor is also quality. From everyday life you now that quality of free things can really vary. But if you pay for something you expect that is has high quality and that you get the best experience.
If you are lost somehow with such things like paid adult websites. You should visit some review site. And here comes our first tip. Welcome to Pay Sites Reviews.