This is a flared brass fitting that is required for hooking up the Blichmann BeerGun to a source of carbon dioxide. This fitting screws into the CO2 valve side of the BeerGun and can adapt it to a standard 1/4" swivel nut, which can be clamped and attached to your CO2 source. Note that this piece is included with the BeerGun accessory kit. However, for those who already own all the other necessary parts that come with the BeerGun accessory kit, this fitting is a great buy and a must have!