Existing understanding, called prior knowledge, becomes the roots in synthesis of new understanding, called insight (i→14, 145). As justification, this is a form of development of understanding.
This shouldn’t be confused with knowledge in general, as, when we read from books, we pick up information. This information is not necessarily understood. It becomes understood when it interacts with other understanding, often in this case by being rooted (the information, understood as beliefs, becomes justification or is justified).