This is a current measuring "shunt", for measuring very high currents like the starter current or the output of the alternator. Most likely it is designed to work with just about any standard voltmeter. This device gets installed in the circuit path, either permanently of just temporarily. Note the buss bar that can be swiveled in or out and secured in position. When installed in the IN position, the path is virtually a direct path, no resistor in the path, That allows normal operating situation. But if you want to measure the current, you swivel the buss bar out and that puts the resistor, with the wavy cooling fins, in the current path. Then you measure the voltage across the resistor with a voltmeter/ multimeter. The alligator clips should clamp onto the barrel ends of the resistor, and the other ends of the leads go to a voltmeter. The resistor is surely a very low resistance, like maybe 0.01 Ohms. If, for example, it IS 0.01 Ohms, then 20 Amps flowing through it would measure as 0.2 Volts ( Volts = Amps X Ohms). Note that this low resistance allows the measurement to be made while only subtracting 0.2 V from the supply being delivered to the load. The neat thing about this device is that it allows the user to quickly and easily switch from measuring to not measuring and back, simply by swiveling the buss bar in and out. Otherwise you would need to undo the wiring to insert an ammeter and the again undo and redo the wiring to remove the ammeter. External shunts, such as these are very common for measuring high currents, but I have never seen one with this swiveling buss bar.
I am guessing that the resistor value is in the tiny stamping on one end of the resistor. Can you read it?
Tom