Psychohistory is a discipline that tries to explain historical events based on the psychology of individuals and even whole nations that were involved in those events.
For example, a psychohistorian might try to explain the Holocaust through a psychological examination of Adolf Hitler and also of the German people as a whole.
You could say that the discipline began with and was based on the work of Freud.
Psychohistory uses the laws of mass actions and it can predict the future, but only on a large scale, on a small scale being prone to errors.
Psychohistory works based on the principle that the behavior of masses of people can be predictable if the mass is very large in number (such as galaxy population, which is about a quadrillion of people). If the mass is greater in number, the future can be more predictable.