A hay baler is a machine that bales hay. It does this by compressing the hay into a rectangular shape using two metal plates. The plates are then fastened together with twine.
The process of tying the twine around the baled hay is known as tying off. This is typically done by hand, though some newer machines have automated the process. To tie off by hand, the operator inserts their arm into the opening at the end of the baler where the baled hay will come out. They then take hold of the twine and wrap it around the baled hay several times, before knotting it off.