A conspiracy theory is a story that says that a group of people ("conspirators") have agreed ("conspired") to do illegal or evil things and hide them from the public. Conspiracy theories usually have little or no evidence. Many conspiracy theories say that certain historical events were actually caused by such conspirators.
Some conspiracy theories are about the Apollo moon landing, the 9/11/01 terrorist attacks, and the assassination of JFK. Other examples not relating to specific events are the Illuminati, Freemasonry-related conspiracy theories, and chemtrails.