Why does Atticus go to the jail in Chapter 15 of To Kill a Mockingbird?Part Two of the book.When Atticus goes to the jail and runs into Jem and Scout.

Atticus shows his unflinching courage and determination when he goes to the jail by himself in Chapter 15 of the Harper Lee novel, To Kill a Mockingbird. Earlier in the day, he had been paid a visit by a group of citizens, including Dr. Reynolds and Mr. Avery. They spoke quietly, and Scout was uncertain of their intentions. But we can infer that they had appeared to warn Atticus that there was talk about the town of taking Tom Robinson from the jail that night--a lynching in Maycomb.
So, Atticus headed to the jail to support Sheriff Tate if the need arose. Soon, the group of men arrive. Atticus tries to nonchalantly talk the men into leaving, but he is unsuccessful. Futhermore, Sheriff Tate has been called out "on a snipe hunt."

"... Heck's bunch's so deep in the woods they won't get out till mornin'."

Atticus is left alone to confront the lynch party by himself. When things appear hopeless for Tom and violence appears likely, Jem, Scout and Dill come to the rescue, and the men's deadly resolve melts before Scout's innocent and genuine amicability.
Tom remains safe, Atticus breathes a sigh of relief, and Mr. Underwood surprises them all by announcing that he had them covered--with a double-barreled shotgun--all along.