That is for 'turning' hay so that it dries evenly. When grass is cut, you want the water to evaporate evenly, so-as-to leave as many of the nutrients there. The water needs to be removed, for longevity. If you don't remove the water, then the hay can 'mould' and the livestock won't eat it.
When fresh hay is cut, the top dries, but the bottom part (due to original weight) sits down deep in the cuttings, and doesn't dry near as fast. This device 'fluffs' it up, turns it over, so that it is 'looser' for more equal drying.
That channel, Northmen, has a lot of cool stuff on there.
Those guys are part of the inspiration for me to do the mass Swedish hand forged axe order we did on here a while ago.