The main cable is the electrical equivalent of an umbilical cord, and connects the console to the rest of the instrument. On theatre organs equipped with traditional relays, the main cable will contain a wire for every note of every manual (plus second touches), a wire for every stop, and (depending on the relay design) a wire for every note of every coupler.
A moderate-sized instrument can therefore have a main cable with over 500 wires in it. What is remarkable is that the cable construction (usually varnished and double cotton-covered copper wire) is so robust that cables over seventy years old are still functioning reliably. Conversely, main cable problems can be very difficult to overcome.
One of the advantages of modern electronic relays is that the number of wires can be drastically reduced, even to the extent of a piece of computer ribbon cable with 23 ways or the ultimate, a coaxial cable.