In any kind of online usability study, you may include some questions at the beginning to automatically screen out participants who don't meet basic criteria for the study. The idea is that if someone doesn't answer the screening questions in a desired way, they are not asked to participate in the study. It's simply a way to ensure that you are only reaching the intended audience.
The types of screening questions to use will differ depending on the Web site you're testing. In general, however, consider whether to screen out at least two groups of people: