A Phantom Zone projecter is a device that was originally used by Kryptonians to instantaneously beam prisoners into Fort Rozz (prison) in the Phantom Zone. Hank Henshaw later re-purposed it to beam himself, Lillian Luthor, and Kara Danvers onto Rhea's spaceship.
The Phantom Zone projector was used by the Kryptonians to transport prisoners into Fort Rozz (prison). Clark Kent would later store a projector in the Fortress of Solitude amongst his various artifacts from Krypton.
Due to the invasion of Earth by the Daxamites under Rhea's leadership, Cadmus and the D.E.O. agreed to work together to save Lena Luthor and Mon-El from Rhea. Kara Danvers, Lillian Luthor, and Hank Henshaw went to the Fortress of Solitude and used it to board the Daxamite cruiser. After locating her daughter Lena, Lillian betrayed Kara and beamed herself, Lena, and Hank back to the Fortress of Solitude leaving Kara and Mon-El behind. Kara then revealed that Winn Schott bugged Hank Henshaw as a precaution against betrayal. Kara activated the bug forcing Hank to reactivate the projector to beam Mon-El out of the spaceship.