Everyone knows what discrimination is. To treat others badly compared to another based on some (usually immutable) characteristic. May people are aware that discrimination is wrong — done by those only who are obviously racist, sexist, transphobic, etc. But is that what discrimination really is?
The thing is, everyone discriminates. You might not be homophobic, but you might treat a woman who chose not to wear makeup differently than one who did. You might not refuse to pick a woman to be on your team, but think twice a man applies to be a babysitter. And all of these treatments are based in prejudice; discrimination is not just hate, but could be as simple as picking a boy over a girl to play a game.
Because of some (unrelated) characteristic, people tend to think that there is some connection between those who share that same attribute. Stereotypes created through justification, or pretending that it isn’t a stereotype is how discrimination spreads.