A simple quarter inch (aka 1/4", 6.35mm or even sometimes called 6.5mm) stereo headphone (TRS - Tip Ring Sleeve) type socket, once upon a time everybody used 1/4" plugs on their headphones, these days I suppose it's more common with guitars and musical instruments in general.
Pass through for all three connections when the plug is not inserted, when the plug is inserted the connections on the right side (looking at the socket from the plug end) get connected to the plug, and the ones on the left side get disconnected (eh, this is obvious when you look at it).
So for example you can put your "speaker" on the pass through pin, and the sound output to the normal pin, with nothing plugged in, the sound passes through to the speaker, plug in and the speaker pass through pin is disconnected.
Supplied with plastic nut and washer for panel mounting if you want (solder wires to the pins), or PCB mount it, choice is yours.
Depending on batch may or may not have a decorative gold coloured ring around the socket hole. If you specifically want or do not want a gold-ringed variety then ask me before buying, what is available varies, otherwise, you get what I got!
Can't say much more about this, not really very exciting, but if you need one, you need one.
Be sure to see my other audio plugs and sockets listings
All of our products are shipped from Christchurch, New Zealand using CourierPost for NZ, and Airmail for international.