Who is Sisyphus? Why is he rolling a rock up the side of a mountain?

Sisyphus is a character in Greek mythology who is condemned to spend his life rolling a boulder up a mountainside. There are some variations on the Myth of Sisyphus, but it is generally told that he was the king of Corinth. He was exceptionally clever, in some versions described as a highwayman, and is even said to have cheated Death! For being so bold, the Gods set a punishment for Sisyphus that he could never overcome. He must roll a heavy boulder up a mountainside, only to have it roll back down once reaching the top. He must perform this task over and over for all eternity. 
Sometimes people will call a repetitive and futile action a "Sisyphean task," referencing the Greek myth.
https://www.britannica.com/topic/Sisyphus

https://www.nyu.edu/classes/keefer/hell/camus.html