Duffer - -made a good suggestion. But if there is NO voltage (DC - both on and off ignition), then change the meter setting to ohms and repeat Ignition off. If there is no resistance, then it is probably a ground cable. Looks like a large Black cable. Many vehicles have several ground cables or straps. battery to frame, frame to engine, frame to dash, etc. Also be aware some meters use a small amount of voltage to measure resistance. But that doesn't look like a late model tractor with a computer.
All that said, looks (at first sight) to be a ground cable to the motor for the starter. If it is and still runs the starter, It may have found another ground. Like the engine mounting blocks for somewhere else. Good luck.