What, in your opinion, represents the constant struggle of rolling the stone in "The Myth of Sisyphus?"
In his preface, Camus states that he wrote "The Myth of Sisyphus" in order to resolve the idea of suicide. Even in a seemingly meaningless world, Camus insists that suicide is not "legitimate"; in other words, suicide is not a rational solution to the problem of meaninglessness:
. . . even if one does not believe in God, suicide is not legitimate. Written fifteen years ago, in 1940, amid the French and European disaster, this book declares that even within the limits of nihilism it is possible to find the means to proceed beyond nihilism. In all the books I have written since, I have attempted to pursue this direction. Although “The Myth of Sisyphus” poses mortal problems, it sums itself up for me as a lucid invitation to live and to create, in the very midst of the desert.
That being said (written), the rock rolling down the hill represents any element of life that is a struggle: anything that suggests that life is meaningless. If every task simply ends up being negated (pushing the rock up and the rock inevitably rolls back down) then the task is 'inevitably' meaningless. Not to mention, this allegory suggests that all of our actions end up with the same result: being stuck in the same position as when we started. Nothing ever really changes; therefore, every effort (pushing the rock up the hill) seems meaningless because each effort is negated (rock rolls back down).
Pushing the rock up the hill does seem to be nothing more than a meaningless punishment. But Camus insists that we imagine Sisyphus happy. By scorning the Gods/fates that gave him this punishment, Sisyphus owns the rock; he takes his fate and uses it as an invitation to be creative. Perhaps he might make a game of pushing the rock. He is the Absurd Hero because he takes an Absurd situation and gives it meaning . . . somehow. So, from Camus' perspective, the pushing of the rock up the hill is Sisyphus' punishment (seemingly meaningless) but it is also an opportunity to find meaning. Camus says "there is no sun without shadow" implying a duality of opposites; with every struggle there is the opportunity to see it as meaningless and/or meaningful. By analogy with the real world, Camus would implore the individual to find meaning in any task, even if it seems as futile as pushing that rock up the hill. The struggle is both futile and fertile; the interpretation is up to the individual.
Each atom of that stone, each mineral flake of that night-filled mountain, in itself forms a world.