The Osterhagen Key was a device that was held by UNIT by the time of the New Dalek Empire's invasion of Earth.
After Earth fell to the Daleks, General Sanchez of UNIT gave the key to Martha Jones along with the prototype of Project Indigo to escape. However, former Prime Minister Harriet Jones told Martha Jones not to use the key under any circumstances . (DW: The Stolen Earth)
The purpose of the key is among UNIT's most closely guarded secrets. Jack Harkness of Torchwood 3 was able to learn the details of another top-secret UNIT initiative, Project Indigo but he had no knowledge of the key. (DW: The Stolen Earth)
Osterhagen is an anagram of Earth's gone. The is a reference that Earth has been stolen by the Daleks. The actual spelling of the name of the key is unknown, but it was pronounced differently during the episode The Stolen Earth by various characters.