Categorising people is convenient, because it keeps things simple. Moreover, it allows you to assess new situations. We learn at an early age to make all kinds of connections between groups of people and the way they behave. For instance, women are caring, elderly people are not handy with smartphones and computers. Categories can be useful, for instance to avoid danger: if you see certain people coming, you may want to cross the street.
These processes take place in your head, automatically, on ‘autopilot’, as they say. But there is a good chance that some of the connections you have made between ‘categories’ and ‘behaviour’ stick. Your preconceived notions become unshakeable. You think you know for sure. This is how preconceived notions, or prejudices, are created.
With prejudice, you assume that all members of a specific group will behave in a certain way. Maybe you have learned consciously or unconsciously that women are bad drivers? Your preconceived notion may be that every woman - including this one, whom you do not know - is incapable of parallel parking.