For example, many people know Justin Beiber but Justin Beiber doesn't know as many people.
You might go to Justin Beiber and ask him who his friends are. Then you ask Justin Beiber's friend who his friends are. Justin Beiber's friend would probably have more friends than Justin Beiber because Justin Beiber is more popular so might be busy while his friend is just Justin Beiber's friend and is unpopular. Justin Beiber is popular, so you know him. You might not know his friend, who might be unpopular.
The underlined section you pointed out means that the question and survey is somehow unfair because you would probably choose 100 people from you circle of friends, popular people you know, or acquaintances.
Student or Learner