Expert determination is like asking someone who is really good at something to help settle a disagreement. Let's say you and your friend can't agree on whether a cartoon character is a cat or a dog. You both ask an expert in animals, like a veterinarian, to help you decide.
The expert will look at all the evidence and make a decision. Their decision is final and everyone has to agree with it, even if they don't like it. It's sort of like asking a teacher to be a judge and make a decision in a game where you and your friend can't agree on the rules.
Expert determination is used in real life when people can't agree on something and they want an unbiased person who knows a lot about the subject to make a decision. This can be used in many different situations, like when businesses are arguing over a contract, or when people are trying to settle a legal case.