Windmills are structures build in the end-game of Valheim. They are used to refine barley into barley flour that is used in the best food items in the game. Windmills must be constructed near an Artisan Table that can be constructed after defeating Moder.
Windmills will refine your barley automatically and can be loaded with 50 barley at a time. You can collect the flour whenever you want by pressing USE on the output chute. Once the entire batch is complete, the flour will spit out the chute and sit on the ground for you to collect it later.