You can find Diamond’s discussion of what he calls the “Great Leap Forward” beginning on p. 39 of Guns, Germs, and Steel. The Great Leap Forward was a time when human culture really took off. Before the leap, humans did not create much art or technological innovation. During the leap, that changed. This was a time when human artistic expression and technological innovation boomed. In a sense, you could say it was when we started to become fully modern humans.
The Great Leap Forward eventually affected all of the human beings living in the world. Diamond says, on p. 40, that we do not really know exactly how that happened. Some people think that humans in Africa underwent the leap and then spread out across the world while others say that the leap occurred at the same time in all human populations. Diamond says that “the issue remains unresolved.”
As to what caused the leap, we are also unsure. There is a consensus that the basic cause of the leap had to do with language. Diamond argued in a previous book that this came about because of anatomical to the voice box that allowed humans to speak as modern humans do. Other scholars have argued that the organization of the human brain changed and that was what allowed people to develop modern languages. Either way, it appears that the Great Leap Forward was catalyzed by the development of language.
So, the Great Leap Forward was a boom in human artistic expression and technological innovation. It came about because human beings became capable of modern language and it eventually affected all human populations on Earth.