That's the starter relay. One heavy wire goes to the battery. The other one to the starter. The 2 small wires are to ground and the other to the start position on the key.
Depending on the year (does it have breaker points?) one wire will bypass the coil resistor when in the start position to give a full 12 volts to the coil. When the key is released, the coil only gets between 6 - 8 volts to prevent burning of the breaker contacts.