When learning about something, it could be useful to have a glimpse of it or a first pass over it from one angle then later on revisit the same concept again from a different angle or have a second pass over it from a different source. For instance, you might read about a specific concept from a book about ecology. You may later read about the very same concept in a textbook about biology. The terminology would probably sound familiar during this second pass. You would probably understand the concept more fully now while you might have not been able to grasp it completely during the first pass while you were reading about it in the ecology textbook.