Okay so imagine you have a toy and you really like playing with it. But one day, someone else comes and takes that toy away from you. You might feel upset or angry, right?
Now let's talk about Gandalf. Gandalf is a very powerful wizard in a story called "The Lord of the Rings." He has a special job to protect some important things and people. But one day, some powerful people thought Gandalf was doing something wrong and they took away his job.
But Gandalf didn't think he did anything wrong. He wanted to prove that he was still a good wizard and deserved his job back. So he went to a big meeting called a trial. At the trial, Gandalf had to explain why he did what he did and why it wasn't wrong.
It was like a big game with rules: Gandalf and the people who didn't believe him had to argue, and someone called a judge listened and made a decision. The judge was like a grown-up, who makes sure everyone plays fair and tells the truth.
After a long talk, the judge listened carefully and decided that Gandalf was actually a good wizard and he deserved his job back. Gandalf was very happy and then he went back to doing what he loved - protecting the important things and people he cared about.
So basically, a trial is like a big game where someone who is accused of doing something tries to prove that they didn't do anything wrong, and someone else decides who is right and who is wrong. And in Gandalf's case, he won the game and got his job back.