At the beginning of Chapter 15, Scout recognizes that something important is up. Tate, Deas, and others are gathered outside Atticus' house:
In Maycomb, grown men stood outside in the front yard for only two reasons: death and politics.
Atticus is representing Tom Robinson who is being moved to the county jail the next day. However, Heck Tate and Mr. Link Deas are a bit worried about leaving Tom in Maycomb's jail in the meantime. Deas, in fact, thinks Atticus has nothing to gain by representing Tom. They are worried for Atticus and his family, but also for Tom Robinson. Atticus adds that it is unlikely that anyone will come after Tom, that he should be fine in the Maycomb jail for just one night.
Their worries are confirmed. The next day, a mob of men come to the Maycomb jail, presumably to attack and/or kill Tom Robinson. Atticus had prepared for this and, with the naive but effective help of Scout and Jem, gets the mob to disperse.