Threshing is used to get seeds/grains from things.
To thresh you need a grain flail made with 1 slender tree trunk and leather.
You can thresh harvested rye and barley when harvested it has to be on the floor and you gain 2-3 grains from 1 barley.
You can do this with tunips and other veg/plants but you MUST harvest it first with Alt+a and select harvest and when its on the floor walk on top of it and use Alt+a and thresh with alt+a you cant do this with berries just veg and plants.