At the end of The Myth of Sisyphus, Camus meditates on the “absurd hero” Sisyphus, a character from Greek mythology. Sisyphus was a crafty man who repeatedly betrayed and disobeyed the gods. As punishment, he was sentenced to an eviternity of rolling a huge rock to the top of a hill. Every time he got the rock near the top, it would roll back down. Sisyphus would then have to walk back down the hill and start pushing the rock up again.
Camus' meditation centers on Sisyphus' mindset at the times he walks down the hill to get the rock after it rolls down. During these relatively leisurely moments of reprieve from pushing the rock, he can reflect on his condition: “I see that man going back down with a heavy yet measured step toward the torment of which he will never know the end. That hour like a breathing-space which returns as surely as his suffering, that is the hour of consciousness.”
This, Camus muses, is when Sisyphus can think about his horrible state of existence. It's in the reprieve that Sisyphus has the leisure to see the acuteness of his quintessentially absurd existence: The aspiration to get the rock to the top and its predestined frustration. The myth is tragic, Camus explains, but only because its hero, Sisyphus, is conscious of the absurdity of it all.
But Camus also says that Sisyphus is happy because he is aware of his tragic situation. He understands his fate and understands he can't avoid it, so he ceases to expect that he'll ever succeed in getting the rock over the top. In other words, he negates all hope. It's frustrated hope that makes the punishment hard, Camus says, so if hope is negated, the torture ceases. And then Sisyphus can roll the rock up the hill, concentrating all his effort on it, resigned to the fate of it rolling back down, but happy because he stands above fate by recognizing it for what it is: absurd.