Headphone adapter. Specifically, it's a 3.5mm female to 1/4 inch (6.35mm) male stereo (TRS) audio jack adapter. This adapter allows you to connect headphones with a small 3.5mm plug to equipment with a larger 1/4 inch jack, commonly found on professional audio equipment such as amplifiers, mixers, and audio interfaces. The two black rings on the plug indicate that it is a TRS (Tip, Ring, Sleeve) connector. This connector provides two-channel audio (stereo) and a ground connection.