Full Idea
Suppose a demon were to say to you, "This life as you have lived it, you will have to live once more and innumerable times more". …Then the question in each thing, "Do you desire this once more and innumerable times more?" would lie across your actions.
Gist of Idea
Imagine if before each of your actions you had to accept repeating the action over and over again
Source
Friedrich Nietzsche (The Gay (Joyful) Science [1882], §341)
Book Reference
Nietzsche,Friedrich: 'The Gay Science', ed/tr. Kaufmann,Walter [Vintage 1974], p.273
A Reaction
If you were stuck in nihilistic indifference, this thought might not be enough to rouse you from your torpor. If all possibilities in life are boring, repetition cannot pep it up, or make it any worse. But I still love this idea!