How close two sets are is not a universally accepted concept, such as the concept of an even number. Each textbook and course can define their own ways of determining how close sets are.
Therefore, you need at least give us the name of the topic and relevant definitions. I would say that the easiest way is to go through the chapter and try to understand it, even though this may not in fact be easy. Also, this is not a site where people explain topics from the beginning. Once you get some idea, though, feel free to ask questions about the details.