Confirmation bias explains our tendency to overvalue information that confirms our beliefs and ignore information that disagrees with our beliefs. This bias generally affects two things: how people gather information (we generally search for information which confirms our hypotheses rather than trying to disprove ourselves), and how people interpret information (we generally interpret stories or data in ways which upohld our hypotheses). In a sense, it can lead to cases of self-deception, where we are presented with all necessary facts but deceive ourselves into only valuing those which confirm our thoughts. This is an incredibly impactful bias, as is prevents us from objectively and fairly assessing a number of situations, some of which can be markedly important (such as our assessment of the actions of politicians) and some of which can be less important.
Consider the recent U.S. Presidential election. Those opposed to Trump would pay closer attention to his crass comments, because it confirmed their dislike for him. Meanwhile, those opposed to Clinton paid more attention to her email scandal, because it confirmed their preconceived distrust in her. In doing so, each supporter was demonstrating their own confirmation bias.