Dill is made physically, emotionally, and socially upset by the open, sneering racism with which Tom Robinson is treated by the prosecuting attorney, Mr. Gilmer, during his cross-examination at the rape trial.

Dill starts to sob so loudly during Mr. Gilmer's cross-examination that Scout has to take him outside the courthouse to calm down. They sit together under an oak tree. Dill explains to Scout how upset Mr. Gilmer made him:

Mr. Finch didn’t act that way to Mayella and old man Ewell when he cross examined them. The way that man called him ‘boy’ all the time an‘ sneered at him, an’ looked around at the jury every time he answered—

Dill won't accept Scout's explanations that all of this is par for the course. Whatever the normal social conventions may be, Dill is sickened by Tom Robinson's rude mistreatment.

Dill's stomach is upset by what he has witnessed, and he is grateful when Mr. Raymond gives him some coca cola to settle it down. Mr. Raymond notes that Dill has not yet toughened up and accepted the racial injustice inherent to Southern society. Through Dill, a sensitive and innocent child, we as readers get a sense of how truly unjust the system is.

You can find the effect of the trial at the end of Chapter 19 and the start of Chapter 20.

Mentally and physically, Dill is traumatized by the Tom Robinson trial.  Specifically, he is traumatized by the way in which the prosecuting attorney treats Tom when he is cross examining him.  Dill is affected so badly that he has to be taken out of the court room and is almost going to throw up.

Socially, I guess that you can say that it is going to start making Dill be more hardened to the horrible things that happen in the world.  At least that is what Mr. Raymond thinks.