In the 1860's there was an estimated total of about 3,953,761 (3mil) of slaves that were living in America. Around that time the American Civil War was just about to begin, and slaves made up about 13% percent of the national population.
The purpose of this passage is to clarify what a UFO is. The passage tells you that even though UFOs often star in science fiction movies, most UFOs are actually aircraft, kites, clouds, meteors or rocket debris. It also cites a famous case in order to provide evidence for their claim.
