What does the horn relay do in a 1963 Chevy 2?

In a 1963 Chevy II, the horn relay acts as a switch that controls the high current flow to the horn(s). The steering wheel horn button only carries a small current; it's not designed to handle the amperage needed to power the horn's electromagnet. The relay takes the small current signal from the horn button and uses it to activate a larger switch within the relay, completing a circuit that allows a much higher current to flow directly from the battery to the horn. This protects the horn button and wiring from damage.