At the end of Chapter 21 of Harper Lee’s To Kill a Mockingbird, the trial of Tom Robinson, the crippled African American wrongfully accused of raping a white woman, has ended with, as expected, his conviction.  Atticus Finch has done his job, and he’s done it well.  The racially-motivated jury may have ignored all of the evidence in its rush to convict a black, but Atticus has succeeded in exposing the racism that dominated the accusation, trial and conviction of a desperately poor and physically handicapped African American.  If there was no justice in the trial’s outcome, there was, at least, a moral victory of sorts in the obviousness of that outcome.  The town’s black community recognizes this, and appreciates the efforts put into Tom’s defense by a respected white attorney acting out of conscience.  It is for this reason that that community, forced to occupy the balcony because of the racial segregation that rules Maycomb, stands in a show of appreciation and respect for this Caucasian lawyer.  As Lee describes the scene, Scout has been initially oblivious to this show of respect for her father:

“Miss Jean Louise?”

I looked around. They were standing. All around us and in the balcony on the opposite wall, the Negroes were getting to their feet. Reverend Sykes’s voice was as distant as Judge Taylor’s:

“Miss Jean Louise, stand up. Your father’s passin‘.”

When Atticus leaves the courtroom after the trial, the balcony (which was seating for the "colored" people - although Jem, Scout, and Dill are also up there) stands as a sign of respect and appreciation towards Atticus. In the novel, Scout is told the by minister up there with them to stand because her father is "passing by."

The whole balcony stands as a sign of respect to Atticus.