The eternal recurrence is a central notion of Nietzsche’s thought. It supposes that you’d have to experience the same life, with the same events and same experiences, repeated for eternity.
Nietzsche suggests that most people would consider this a curse and that it would require the most impassioned love of life: to crave nothing more fervently than this ultimate eternal confirmation and seal.
The idea is horrifying and paralysing as it carries the burden of the “heaviest weight” imaginable. However, it is also the ultimate affirmation of life, it is the rock the fills the emptiness and weightlessness void of nihilism.