Prejudice means preconceived opinion that is not based on reason or actual experience. The word comes from the Latin "pre" (before) and "judge". It most often refers to an opinion about a person or group of people.
The word "prejudice" is often used when people dislike another group of people that are different from them. They may decide they do not like them because of their skin color (this is "racial prejudice"), religion (religious prejudice) or nationality. These are all serious prejudices which can lead to discrimination, hatred or even war.
Prejudice in popular culture
Judges in a court of law should not be prejudiced when deciding whether someone is guilty. They should have an "open mind" so that they can make a fair decision. For example, if that person is the judge's friend then the judge would be prejudiced because he (or she) would not want his (or her) friend to get into trouble.