Prejudice is when, on the basis of knowing a few things about a person, or even just one thing, we assume that other things must be true about them. It might mean assuming bad things or it might be good things. Either way it's irrational.

What is related to it and is not necessarily irrational is is perhaps bettwer termed a predisposition rather than a prejudice - making provisional judgements based on experience. When I go to a concert or a film or a festival, or decide not to go, that involves acting on the basis of a provisional judgement. Picking food from a menu in a restaurant is a similar process. If I've had a bad experience with some kind of dog, I'm going to be wary the next time I come into contact with a similar dog.

Inevitably the same kind of thing enters into our relations with other people.

The important thing is to avoid being trapped by these kind of provisional judgements, and alllowing them to turn into rigid rules that limit our range of experiences, or cause is to put up barriers against other people.