What does it mean to compare new information against what you already know?

What does it mean to compare new information against what you already know?
Many people have previous knowledges about a certain subject. When someone encounters new information, it may contradict, support, or provide a different perspective on what he or she already knows. To compare new information with the old one, one should note what relationship exists, contradictory, augmenting, or something else? It means to find similarities, differences between what you knew and the new information you acquired.