What is a Maycomb County Jail ?
Well, there's not really any such thing as "a Maycomb County jail"; it's the Maycomb County jail. Remember, Maycomb is not only the name of the town in which the Finches live, it's also the seat of the county by the same name. That means that both the county jail and the county courthouse would be located in the town. This county jail is where Tom stays during the course of the trial, and where Atticus guards him the night before the trial begins.
It becomes a scene of tension and conflict when an angry mob appears, demanding that Atticus release Tom to them. Atticus refuses, and the mob grows increasingly restless and frustrated. Scout and Jem reveal themselves, and one man threatens Atticus, telling him he has 15 seconds to get rid of the kids. However, Scout diffuses this difficult situation by correctly identifying Walter Cunningham's father in the crowd. Her talk of his legal problems, and her innocent reminders of how her father has helped his family, shames him into calling away the rest of the mob. Each man realizes that the children have behaved more courageously than themselves.
Thus, the jail becomes a symbol of courage and cowardice. The scene that unfolds that night demonstrates the power of a mob, and how many people will let themselves be swept away by a crowd, even if its actions are contradictory to their own morals and values.