This sentence appears at the very end of Chapter 17. Scout sees that Jem "seemed to be having a quiet fit. He was pounding the balcony rail softly and once he whispered, 'We've got him.'" Scout realized that Atticus was trying to show that Ewell could have beaten up Mayella instead of Tom, whose paralyzed arm would have made it impossible to inflict the bruises on the right side of Mayella's face. But Scout is not convinced that Atticus has won his case. When she says, "I thought Jem was counting his chickens," she is referring to a folk saying that used to be common in America when more families lived on farms. The saying was: "Don't count your chickens before they're hatched." If you have hens sitting on a dozen eggs, that doesn't mean your going to get a dozen chicks. The outcome of the novel shows that Scout was right. Just because Tom should have been declared not guilty didn't mean that he was going to be declared not guilty in that time and that place.
This is a good question. First, it is important to know the whole saying: "Don't count your chickens before they hatch." This means that you should not be too confident that all things will go your way. In other words, if you had ten eggs, that does not mean that you will have ten chickens.
Scout say these words in the context of the trial of Tom Robinson. This is where Atticus brilliantly shows that Bob Ewell is left-handed. This shows that he could have beaten up Mayella, because the bruises on Mayella's face were on her right side. Jem believed that this was a slam dunk piece of evidence to ensure Atticus's victory and Tom Robinson's freedom.
Scout, seeing Jem, say these words. She is saying that Jem is too confident. Scout thought that Atticus's argument was good but not airtight. She says:
But Tom Robinson could easily be left-handed, too. Like Mr. Heck Tate, I imagined a person facing me, went through a swift mental pantomime, and concluded that he might have held her with his right hand and pounded her with his left.
Unfortunately, the children did not realized that in this court of law what mattered most was the color of your skin.
Scout is using a form of the idiom,
"Don't count your chickens before they are hatched."
This idiom is most often used when one thinks that someone is unrealistically optimistic about a situation. Such is the case when Jem and Scout are in the courtroom listening to Atticus in court. Jem expresses his confidence that Atticus has succeeded in proving that Tom could be innocent of the crime with which he is charged. The point Atticus was making in court had to do with whether or not the person who attacked Mayella was left or right handed. Atticus was attempting to prove that another man could have been the one to create the marks left by the attack on Mayella rather than Tom. In Jem's opinion, Atticus had proven without a doubt that Tom could not have been the attacker and would be freed. Scout however, feels that Tom could still be found guilty and so she feels that Jem is too quickly jumping to conclusions.