This is a broad question. There are many things that happen at the trial.
The biggest point is that Tom is found guilty by a racist panel of jurors. Atticus does a brilliant job in defending Tom, but it does not convince the jurors.
Also during the trial, the character of Atticus shines. Every step of the way, he is kind, compassionate, and respectful towards Bob Ewell and his daughter Mayella. Atticus shows that he takes no delight in proving anyone wrong, but he has to stand for justice.
During the trial we also find out that Tom is handicapped. He has little use of one hand. The further proves that he could not have committed the crime.
During the trial both Jem and Scout do some growing up. They realize that Atticus is not an old weakling, but a courageous man whom Maycomb needs because he is a man of courage and integrity. He will protect innocent people. This is why one of my favorite quotes in the book is when Reverend Sykes tells Scout to stand up for her father.
I looked around. They were standing. All around us and in the balcony on the opposite wall, the Negroes were getting to their feet. Reverend Sykes’s voice was as distant as Judge Taylor’s:
“Miss Jean Louise, stand up. Your father’s passin‘.”
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Basically, Tom was accused of forcing himself on Mayella Ewell in an improper way. The town has a lot of racism. Blacks are still segregated, including in the courthouse where they have to sit in the balcony seats. Tom is doomed to being found guilty by an all white jury. He never really has a chance. The accusations are the fantasy of Mayella, most likely a neglected child who cannot figure out how to handle her fantasies. Scout, Jem and their father believe Tom is innocent. Scout's father, Atticus, is the defense attorney. He takes abuse from the town folk for defending Tom and one night they come to threaten him with torches in hand and suggest that he drop the case. Atticus Finch is not one to give in though. He skillfully defends Tom to the point where there is NO DOUBT that he is innocent but the jury inexplicably decides that he is innocent.
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The jury find Tom guilty.
Thanks for the reply but needed a bit more detail