If you've ever tried machine learning, it's easy to understand what a prejudice is: inference based on learning on biased data. Or sometimes the data doesn't even need to be biased: your stereotyping may be factually true on average. It's just that we should not make assumptions based on averages, especially when they involve people.
Well... I mean if I'm going through an area known for violence and robberies and someone is following me, my prejudices might keep me alive. That's an extreme example of course but I think humans use their instincts every day. Calling it prejudices make it sound negative but I think it's just part of our survival instincts.
And just like anything else it can be used negatively, or positively. As long as we don't use our prejudices premeditated to harm others I think it's a survival tool. But when we use them to defame groups of people who are nowhere near us then it can become negative.
