It's the adapter on Bose Quiet Comfort 15 headphones (QC15), and Bose Quiet Comfort 2 (QC2). The plug and jack on the adapter are all 3.5mm.
It's a replacement for the adapter for your BTunes for Bose QC15/QC2. You purchase it if you lose one.
When you purchase BTunes QC15, it comes with this adapter. You don't need to order it separately.