Usually used to extend a speaker or headphone cable (which uses 3.5mm jacks). 3.5mm headphone jacks are found on laptop mobile phones mp3 players Hi Fi systems and TVs among other devices. If your cable from your stereo headset is not long enough then this cable is perfect for you.
3.5mm Jack (headphone jack) to 3.5mm socket - extension Gold plated connectors
Stereo connections have two black rings on the jack connection. Should your headset or cable have 3 black rings you need a 4 pole connection see QS: 008446