The justification of a belief is all the beliefs that are taken as sufficient reason for holding the belief. That belief is said to be understood.
What counts as sufficient reason to hold a belief is completely up to the holder of the beliefs. Critically examining justifications is a way to develop understanding.
For example, suppose you believe that every swan is white. Your friend says that he has a picture of a cool swan. You may believe that your friend’s cool swan is white. If you justify that belief with the belief that all swans are white, then your belief is understood; you understand that your friend’s swan is white. Now your friend shows you the picture of the cool swan, and it is black. You believe this, but it makes you consider what your justification for believing that it was white—that all swans are white. Evidently, this isn’t the case, so now there is an understanding that some swans are white and some swans are black.