This is a 3.5mm Stereo Female Jack to 3.5mm Mono Male Plug Audio Converter / Adapters, as pictured.
Why do you need this? This is a perfect converter adapter for when you require a MONO earbud, but want to use stereo. This adapter will plug into your mono device, and our stereo earbuds fit right into the adapter. The cost per adapter+stereo earbud actually is lower cost than seeking out a mono earbud (the demand is very low, hence higher prices, so we developed this option which works great.)