An expert witness is like a really smart person who helps tell a story in court. You remember how we talked about the judge and the jury deciding who is right and who is wrong? Well, sometimes the judge and the jury need someone who knows a lot about something to help them make their decision. That person is called an expert witness.
Let's say there is a court case about a car accident. The expert witness might be someone who knows a lot about cars and can explain to the judge and jury what happened and who was at fault. Or if there is a medical malpractice case, the expert witness might be a doctor who can explain to the judge and jury what the doctor should have done differently.
The expert witness is not on either side of the case, they are just there to help explain things. And they have to be really smart and know a lot about what they are talking about. Sometimes they get paid a lot of money to be an expert witness because their knowledge is so valuable.