I see it a little differently. I think it refers to the fact that humans make an effort to create culture though they know they aren't going to be around for ever, though they can enjoy it only for a limited period.
It's something they do in the face of their mortality/though their mortality is against them.
See another thread which relates: Without culture, and the relative freedom it implies, [Albert Camus]
Although life might seem as futile as the labours of Sisyphus pushing the rock up the hill, we reject suicide and instead - in defiance of our fate - we plan, we strive, we create, participate in our shared culture. We continue to search for meaning in an apparently meaningless universe.