is the rear of a typical Honda clock which is separate from the radio
unit. There are four electrical pins. The pin highlighted in red is the
provides "backup" power to the clock in the absence of ignition
current. This is the one that passes through the "backup" fuse.
The "ignition" feed is what makes the clock display turn on when you start the car.
The "headlight switch" signal is the one that tells the clock to dim its display, so it won't be too bright at night when you have your headlights on.