Jem witnesses discrimination throughout his childhood as a result of Jim Crow laws.
Jim Crow laws were laws that discriminated against African Americans by forcing them to live in segregation. Under these laws, they were not allowed to go to school, live among the whites, or go to the same churches. They also did not get fair treatment in the legal system, even though they were supposed to. Jem saw all of this.
One example of Jim Crow is Jem's school. There is not a black child in it. In fact, the blacks in Maycomb cannot read, except a handful. Atticus saw to it that Calpurnia, his cook and housekeeper, was educated. She taught her son. When Jem goes to the First Purchase Church, the African American church, he sees how few people there can read. Calpurnia explains this to Jem.
"Can't but about four folks in First Purchase read... I'm one of 'em."
"Where'd you go to school, Cal?" asked Jem.
"Nowhere. Let's see now, who taught me my letters? It was Miss
Maudie Atkinson's aunt, old Miss Buford-" (Ch. 12)
Jem is aware that this is unfair. He is also old enough to understand that things are unequal in Maycomb, and that African Americans are treated unfairly.
Jem's greatest lesson in unfairness is the trial though. Unlike Scout, who is younger, Jem follows the trial with comprehension of the facts, but not understanding of context. He realizes that Atticus has made a strong legal case that Tom Robinson is not guilty. He assumes that he will be acquitted.
It was Jem's turn to cry. His face was streaked with angry tears as we made our way through the cheerful crowd. "It ain't right," he muttered, all the way to the corner of the square where we found Atticus waiting. (Ch. 22)
Jem gets the biggest blow of his young life when Robinson is convicted. It shatters his sense of justice, and his faith in humanity. This is Jim Crow at work, if not in the letter of the law, in the spirit. The system was separate, and not equal. People did not consider African Americans like Tom Robinson equal. For this reason, even a man that could never have been committed the crime could be convicted of it, simply because of the color of his skin. Jem learns this lesson, and the reader learns it to. For this reason, the reader realizes along with Jem, though he or she may not be a child, that this kind of thinking must stop. Jim Crow thinking must end. Sometimes we must see this through the eyes of history, and the eyes of a child.