The Berk Winch is a building built by the Berkians on School of Dragons Island to help raise and lower platforms and objects.
Like a cable winch in the real world, the Berk Winch is built to raise and lower a platform. Objects can be placed on this platform in order to be raised or lowered to the hight they are needed at.
At some point before the time the game takes place, the winch was built on The Lookout on School of Dragons Island. It was run with a spool of cable that could raise or lower a platform.
During the quest "Simple Machines", Hiccup realizes that a pulley system would function better than the cable spool. He and the player construct a large pulley system and place it in the winch, allowing it to perform its tasks even more efficiently.