It is a voltage-sensitive relay that 'protects' voltage-sensitive electronics from voltage that is too high for them. It reacts to high voltage by energizing and shorting its positive lead to ground, thus melting a fuse and cutting off power to the protected electronics. The theory is that no voltage is better than too much voltage.

It is sometimes known as a 'crowbar' circuit, from the idea that you protect the circuit by throwing a crowbar across the incoming line, causing a short circuit that guarantees that the fuse melts and protects the circuit. After all, OFF is safe...

That is a method of preventing electronics developed before electonics coud withstand any voltage that the alternator can reasonably produce. For many cars the OVP is no longer needed.