Imagine you're playing with your toys, and you have lots of different types of toys - dolls, cars, balls, etc. Now, imagine that someone comes along and says to you, "I don't want you to play with the dolls because you're a boy," or "I don't want you to play with the cars because you're a girl." That wouldn't be very fair, would it? That's kind of what employment discrimination is like.
In the United Kingdom, there are laws that say it's not okay for someone to be treated differently at work because of things like their race, their gender, their sexual orientation, their age, or their disability. These laws are there to make sure that everyone has an equal chance to get a job and do well at work, no matter who they are.
So let's say that you grow up and you want to get a job. Let's say you're really good at fixing things, so you want to be a mechanic. But when you go to apply for a job as a mechanic, the person in charge says to you, "I can't hire you because you're a woman," even though you might be just as good at fixing things as a man would be. That would be employment discrimination, and it's against the law.
Another example might be if you're at work and someone says mean things to you because of your skin colour, or because you have a disability. That's also against the law. Your employer is supposed to make sure that everyone is treated respectfully and fairly, no matter what they look like or how they talk or what they can or can't do.
If someone does discriminate against you, there are things you can do. You can talk to your boss, or if you don't feel comfortable doing that, you can talk to someone else in your workplace like a union representative. You can also go to a place called an Employment Tribunal, which is like a court where people go to sort out problems they're having at work. There are people there who can help you and listen to your side of the story.
So the bottom line is, employment discrimination is when someone is treated unfairly at work because of who they are, and it's against the law in the United Kingdom. Everyone deserves a fair chance to do well at work, no matter who they are or what they look like, and there are people who can help if things aren't going very well.