It is always difficult to choose people on a jury. Every potential juror has a history, education, and daily life which influences their attitude towards everything. And this set of experiences varies from person to person. Show a person a red rubber ball and one person will smile, thinking about enjoyable times in the playground. Another person might grimace and say "get that thing out of here" because, for them, that ball is a reminder of being pelted in dodgeball in gym class and going home with circular welts on the body. Same red rubber ball, very different reactions.
Considering that each person likely has a different perspective and history what are they to do in a court? First, of course, is whether the person has any direct knowledge of the people or events in the case. In such cases, they might be prospective witnesses but they should not be members of a jury. The prosecutor is going to want to find people who will come back with a verdict in their favor and the defense attorney is going to want to find people who will come back in their favor.
This is a problem because everyone will have biases. A bias is a feeling towards something, a first impression and reaction towards something. But it is only the first feeling -- it may change as things happen or they learn more. A child is initially scared of a vacuum cleaner because it makes so much loud noise. As they get used to the noise and watch what is done with it, they may start to appreciate it when the floor becomes cleaner and the air fresher. It might go the other way and they may dislike it even more as they get older and they are given the task of being the person to push the vacuum cleaner from room to room. But show them a vacuum cleaner and make it clear that it is not there for them to use and they will be okay with its presence.
A prejudice is what the word breaks down into -- a "pre" "judgement". You have made a firm reaction, or decision, with no direct interaction with the current situation or information around the event. Prejudices could conceivably be about a thing or a cause but the word is predominantly used about groups of people. A lot of people have prejudices about athletes. Some think that, because the athletes have better-than-average physical abilities, they must not have intellectual abilities. Even if given test scores, awards, and well-testified examples, the best outcome with a person with such a prejudice is that "that person must be an exception".
So, in the case of a potential juror, everyone has biases. If they favor short people then, if possible, they will be glad to support short people. But, if they are given evidence that this particular short person has done something bad, they will not support them. Or vice versa, they don't like people who dress as if they are wealthy, but when the evidence indicates that this particular suited individual has not done any harm, then they will support them and their position.
If a potential juror has prejudices then they will support, or oppose, no matter what the current circumstance or evidence. Whereas, with biases, they are open (perhaps not eagerly open) to being presented with evidence that will lead to a verdict either direction. The prosecution would love to have people prejudicial in favor of guilty and the defense would love to have people prejudicial in favor of not guilty. Each will try to eliminate jurors prejudiced in the other side's favor. Biases are less important but each would like people biased in their direction -- it would mean less effort to convince.
The "fairest" trial would have all the jurors without prejudices or biases -- working to make a judgement solely on the evidence presented. But all people have a history so awareness is our best hope.
