what tense situation occurs at the jail? How does Scout help her father in this situation?

A mob shows up at the jail to "take care" of Tom Robinson. Despite Atticus’s orders to stay home, Scout, Jem and Dill show up at the courthouse. Atticus refuses to let the mob near Tom. When things get heated, Scout makes her way into the crowd and asks Mr. Cunningham about his boy and his life. The simple and kind gesture of a little girl seems to remind Mr. Cunningham of the folly of his ways. Scout diffuses the hostility long enough for Mr. Cunningham, and the mob, to realize just what they are doing.

Jem, Scout, and Dill sneak out in the night to see if Atticus is alright, because Jem was worried about him. Atticus happens to be guarding the jail in which Tom Robinson was staying in. A mob shows up there to come and take care (kill) Tom Robinson. Jem, Scout, and Dill show up at the courthouse jail when this is happening. Atticus refuses to let the mob get near Tom. As the situation is esclating, Scout starts up an innocent conversation with Mr. Cunningham. She asks about his son, Walter and his life. This simple, sweet, and innocent conversation seems to make Mr. Cunningham think about what he is about to do, or perhaps it put himself in Atticus's shoes. Either way, Scout stops the hostility long enough for Mr. Cunningham, and the mob to realize what they are doing and they head home. Once again, Scout has saved the day.