This happens in Chapter 19, during the trial. Mr. Gilmer, the prosecutor, is questioning Tom Robinson. He is trying to get Tom to admit that he has been convicted of a misdemeanor crime of disorderly conduct because he got in a fight with another black man.
So Atticus does not bring it up himself but he does tell the judge that it is true -- it is in the record. What he's trying to do is make it not seem like such a big deal. Gilmer is trying to play it up as proof that Tom is capable of raping someone, but Atticus is trying to make it seem minor -- he just admits it like it's no big deal. This is a fairly common tactic, at least in books and movies about criminal trials.
As Tom's defending attorney Atticus has the responsibility of dispelling evidence in court. Tom's criminal record has been brought up by the district attorney in order to paint a picture of a black man with a history of violence and criminal activity.
It is Atticus' responsibility to make the jury see Tom's arrest as a minor infraction and not equate it to the level of violence that would prove him capable of rape. He tries to undermine the level of Tom's involvement of the incident so that the jury will not see it as a big offence.
By high lighting that the incident happened between two black men, he ahs already reduced the level of violence because it was alright in the southern white's minds if two black people beat each other up. It would have been very different if the events had occurred between Tom and a white man.