When was it invented?
The screw press was probably invented in Greece in the 1st or 2nd century BC. There is evidence to suggest it had been used during the days of the Roman Empire for pressing clothes.
What is it?
The screw press is a device consisting of two horizontally placed parallel plates, the upper-most of which has a screw apparatus afixed to the top of it. The screw is in turn joined vertically to a stationary setscrew which is joined to the base of the press. This fixed setscrew resists the motion of the rotation of the screw and allows for the controlled movement of the upper-most plate of the press.
The press works by applying human strength downward by using the mechanical advantage that the screw presents.
What is it used for?
In antiquity, these presses were used for pressing linens, grapes for wine-making, and olives for oil. These presses are still used today for oil extraction and paper making, albeit some other sort of mechanical power is applied rather than having the press operate on human strength alone.
The screw press is the foundation for the invention of the printing press.