A short cable that allows wired headphones with the traditional 3.5mm mini-phone connector to plug into the Lightning port of an iPhone. Starting with the iPhone 7, Apple eliminated the analog headphone jack. See mini-phone connector
Lightning to 3.5mm Analog Headphone Jack
Starting with the iPhone 7, Apple dropped the analog headphone jack. In order to use an older pair of wired headphones, this adapter is necessary.