When people have problems that they cannot solve, they sometimes ask other people to help them. In a court case, two sides have a problem they cannot solve by themselves. One person says they did something wrong, and the other person says they did not. They need someone to help them decide who is right and who is wrong.
That person is called the trier of fact. They listen to both sides of the story and look at any evidence, such as pictures or documents. The trier of fact is usually a group of people called a jury. They will listen very carefully and ask questions to make sure they understand everything.
After they have heard everything, they will decide who is right and who is wrong. They will tell the judge their decision, and the judge will make sure everything is done correctly according to the law. So, the trier of fact is like a problem-solver who helps people in a court case.