The notion that the universe has been repeating itself over and over, infinitely, and that time is cyclical. Since the universe (or the matter contained in it) is finite, but time is infinite, the number of permutations into which matter can be converted over time has to be finite and its forms will eventually repeat themselves.
Eternal recurrence was discussed in ancient India and Egypt, and also by the Pythagorean and Stoic philosophers of ancient Greece, but fell into disuse for a long time. Nietzsche, who was a brilliant classical scholar, revived the theory as a method of affirming life in the wake of the “death of God.”
While the idea that all events will eventually reoccur might initially strike a person as the “heaviest weight” (das schwerste Gewicht – see page 4 below), Nietzsche urges people to accept and embrace it as the ultimate affirmation of life.