Imagine you are tasked to push a large stone up to the top of a hill. Just as you near the top the stone slips away and rolls back down the hill. You try rolling it up the hill again, and again the stone slips aways and rolls back down the hill. Again and again this happens.
Clearly this is a very laborious and futile task. It is a “Sisyphean task.”
[Definition] A Sisyphean task is an endlessly laborious or futile task.