The Ku Klux Klan is a white-supremacist organization founded shortly after the Civil War, known in the early days for disguising its members in white robes, conical hats and riding robed horses. (It is believed that the renowned Confederate cavalryman, General Nathan Bedford Forrest--of Forrest Gump fame--was the KKK's first Grand Imperial Wizard.) The KKK first flourished in the 1860s before disappearing by 1880. The group reemerged in the 1920s, and then strengthened once again after World War II. The early incarnations of the KKK specialized in terrorist activities--usually at night-- primarily against African Americans, that included beatings, torture and murder--often by lynching. The KKK is still alive today, with more than 150 chapters nationwide and more than 5000 members.
The KKK comes into play during TKAM when Atticus is reminded by Jem about some previous Klan activity in Maycomb. Atticus corrects Jem, saying that the Klan's target many years before were not Catholics, but a (presumably) Jewish family named Levy. However, Atticus tells Jem that
"The Ku Klux's gone... It'll never come back."
Atticus may have been partially correct in this assessment, since the KKK apparently was silent for nearly two decades before the end of the Second World War. But it certainly proved that Atticus was not infallible, since the Klan would soon rise once again. It is also likely that Atticus made this statement in order to soothe his worried family after receiving the news that there may be trouble at the jail later that night.