Missy's device was a piece of multi-purpose technology owned by Missy.
The device later fell into the hands of Clara Oswald, who was prepared to use it to kill Missy out of revenge for what happened to her boyfriend Danny Pink. The Twelfth Doctor took the device from her and told her that he would save her by doing the deed himself. Instead, Missy was shot from afar by the Brigadier in Cyberman form. (TV: Death in Heaven)
Missy later used a new and upgraded version of the device to create a Time stop on all the planes around Earth, making them appear to the world as "frozen". She contacted UNIT with the device. She also vaporised two UNIT guards with it to make the point that she hadn't "turned good". She later used it to release the planes. (TV: The Magician's Apprentice)
While uniting with many of her previous incarnations to take over the galaxy through hypnotism on the program Battle of the Bands Beyond the Stars, Missy planned to use the device as some part of the hypnotism process. While she began to use it, however, The Master began fighting with her over it, declaring that only he and he alone could control the universe. The other incarnations soon joined in, the group was soon disqualified from competition. (COMIC: The Five Masters)