It's a complex world, and in a free and open democracy ideas from every part of the political spectrum are allowed a voice. With the growth of the internet those voices can reach a large audience. It also means there is an overwhelming amount of information, and we have to decide what to read, and which things to believe.
Humans are well suited for looking at evidence and figuring out a course of action. Our action isn't always the best one, but we can usually find an action that works well enough to avoid being burned, killed, jailed, or suffering some other unpleasant outcome.
Confirmation bias is our tendency to give more weight to information which supports our predetermined beliefs. It is probably hardwired into the parts of our brain which process information. The same precise facts can be given to two individuals on the opposite side of the political spectrum, and they will use that data to build different narratives about the meaning of the information.
As a simplified example, but one which comes up often in political discussion, let's take the generic example of poor people. A person on the Left might make the observation that the poor are oppressed, and that the government should help them out with aid and jobs programs. An observer on the Right might conclude that the poor lack the proper work ethic, and that it's up to the poor to pull themselves up by their own bootstraps.
Both approaches take the same set of facts, but arrive at different conclusions. The solutions are fast, simple, and don't involve the sort of hard research and analysis it would take to dig deeper into the problems of impoverished communities, and come up with practical solutions.
Politics isn't the only area where confirmation bias exists. It has been studied in criminal investigation, in medical diagnosis, and in a number of other areas where people make decisions based on evaluating information. Observers tend to pick and choose facts, and plug them into their existing theory.
This may seem fatalistic and a little depressing. As human beings we clearly have blind spots when looking at facts. A progressive and a conservative can read the same information, and honestly arrive at different conclusions on the meaning. The same is true of a Christian and a Muslim, a rich person and a poor person, or a white person and a black person, The things our parents taught us, the region we're from, our religion, the dominant beliefs of our historical era, peer pressure from friends, our economic status and educational level, and many other factors influence what we believe.
Does this mean we're doomed to go through life with an inability to make logical decisions without our perceptions being infected by our biases? There is little we can do to completely eliminate bias, but there is a great deal we can do to minimize its effect on our opinions and decisions.
Here are a few solutions:
Read and listen to opinions more carefully. When a commentator is giving an opinion we should always ask ourselves: Are these facts true? Do they support the commentator's conclusion? This applies not only to people with whom we tend to disagree, but to people whose beliefs we like. We are likelier to be uncritical of someone we generally agree with, and that interferes with our critical thinking.
Try to find strong contradictory interpretations for any opinion. Our alternative argument should not be a "straw man", easy to shoot down. but the strongest possible contrary view. We should keep our emotions and wishful thinking to a minimum, and examine as many facts that support or contradict an idea as possible.
Please, don't take my word for any of this. Learn how to build strong, well supported arguments yourself, and to evaluate the arguments of other people. Confirmation bias interferes with critical thinking, but it doesn't have to deal it a fatal blow.