Jem, Scout and Dill, wanted to sneak into the courtroom in the crowd, so they could witness everything that was going to happen to Tom Robinson and if their father was going to win the case. They managed to get in without Atticus seeing us. However, since all the seats were taken, they had to sit in the balcony, where the black people were supposed to sit.
Because they sat there, they were able to see everything in the courtroom, especially Judge Taylor.