what is Atticus about to do which places his honor in questions
I assume this refers to Atticus' choice to defend Tom Robinson. Although Atticus is a public defender for all citizens in need of a lawyer, he is also a white man living in Alabama in the 1930's. At this time, black and white people led segregated lives. The Finch family has lived in the area for generations and, at one time, owned a plantation. In other words, the Finches have a high social status and it would be considered "beneath" Atticus to defend a black man, especially because Tom is accused of raping a white woman. The community would automatically assume Tom to be guilty, thus Atticus is seen as defending a rapist.
Atticus, actually being a man of honor, decides to defend Tom because he believes it is the right thing to do. He doesn't just go through the motions, he supports every man's right to a fair and speedy trial based on the Constitution. Although, by Maycomb County standards he is putting his reputation on the line by defending Tom and causing others to question his honor, he proves his worth as a man and as a public servant by providing the same type of defense for the African American as he would for a white man.