Prejudice is a preconceived opinion or assumption without grounds or sufficient knowledge. It is to PRE-JUDGE: To assume we know what we cannot know. It is an attitude, and it comes out as racism, sexism, ageism, classism, etc. It is an attitude that (when unchecked) can significantly impact the way we treat others.
This was a problem in the early church. There was prejudice between Jews and Gentiles, rich and poor, and many other places. The message that the early church had to learn through the apostles teaching was this: They were all created in the image of God, and Jesus died for ALL of them. That is the same lesson we must remember today.