Bias is an inclination for or against one person or group, especially in a way considered to be unfair. It is a belief that some ideas, people, etc. are better than others. When one person is biased towards a certain thing, it is difficult to make a fair judgment. This is why judges should always be unbiased. Bias is always linked to unfairness.
If you have a good impression of something, then you are biased toward it. But if you have a bad impression of something, you are biased against it. If you must make a choice about these two things, you’ll be inclined towards the impression you have a good impression about. So, you won’t consider its disadvantages or weak qualities much.
For example, imagine that you are a teacher, and you see two students fighting. You personally know one of them. So you jump to the conclusion that it was the other student who started the fight. Here you are being biased towards the student you personally know. But, you really have no idea who started the fight.