This is a Universal IR (Infrared) Receiver Extender Cable.

What it’s for: If your cable box, Apple TV, or stereo is hidden inside a TV cabinet or behind a door, your remote won't work because it can't "see" the box. You plug this cable into the back of the device and stick the little red sensor (the dome) somewhere visible. Specs & Details: Plug Type: Standard 3.5mm jack (looks like a headphone plug). Color: Black cable with a dark red sensor head. Condition: Clean and ready to use. Price: Free to a good home! Perfect for someone setting up a clean, "hidden" home theater look.