A Receiving Tube is a hollow tube used to receive a needle during a piercing.
In some piercings (vertical hood, PA, and nostril are good examples), the exit hole (that is, where the needle comes out during the piercing) is in a tight location. As such, the needle tip can easily do collateral damage to surrounding tissue.
To combat this problem, a receiving tube is used. The end of the tube is butted up against the exit hole, and tissues are shielded from the needle by the walls of the tube. Receiving tubes are usually either flat ended, or cut at a diagonal to make receiving in places like the nostril easier.