This is a 3.5mm Stereo PCB Mount socket with a switched pass-through for the left and right channels.
There are two pins for the left channel and 2 for the right, and each is connected to the other, L-L and R-R, when you plug in, that link is disabled (with two pins connecting to the plug you inserted, and two unconnected).
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, but insert a 3.5mm plug and the speaker pass through pin is disconnected leaving just your headphones to get the sound.
5 solder pins, (ground, L, R and the L, R pass throughs).
Can't say much more about this, not really very exciting, but if you need one, you need one.
Of course, not limited to audio usage, 3.5mm connectors are convenient means for connecting up sensors and such to your electronic projects. I've even seen cheap handheld oscilloscopes using 3.5mm jacks for their probes.
Be sure to see my other audio plugs and sockets listings
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