1/4" 6.35mm Female Jack to 1/8" 3.5mm Male TS Mono Adapter Mic Guitar Headset
It is a mono adapter (can be used with mics, electric guitars, mono headset/headphone, etc). This is used to convert a thick 1/4" (6.35mm) plug into the miniature 3.5mm size (i.e. the size that used to be standard on iPhones).