Well, in radio, the CD players, record decks, MiniDisk decks and Jingle / advert playout PC needs to be controlled from the broadcast desk...
In order to do this, the mixer sticks a 5V peak when either of the yellow or Yellow buttons are pressed. (Start or Stop)
When you want to start a new track, or whatever, you hit the button, that sticks a 5V pulse down a cable, the LED in the Opto-Coupler lights up, the Photo-Transistor inside conducts and briefly S/C's the play or Cue button on the CD player and voila
It's all done be remote control instead of having to reach over and press buttons.
Hope that makes things a bit clearer
Dr, Dave
(And yeah, that's me in the picture)