Talking about ATV spreaders, calibration means ensuring that the device spreads the right amount of whatever material you intend to broadcast. Most manufacturers specify how much of their product you need per unit of area, so you can test your seeder on a small pre-measured plot of, say, 200 square feet.
Calculate how much of the material you need for this area and load it into the hopper. Cover the subplot you’ve chosen and check the hopper. If there’s any material left, or you ran out of the material before finishing, adjust the feed rate accordingly.