The Key to Time is an artifact of the Guardians of Time (or possibly the Grace, but either way, beings who are like gods to the Time Lords). It controls the balance of the two (or six) fundamental aspects of the universe. It's nearly always divided into six parts, scattered throughout the universe, each hidden in a way that only a special Tracer created by one of the Guardians can find it.
There are three things which have all been called the Great Key, but these are all Time Lord artifacts, presumably created by Rassilon.
- The Key of Rassilon is the key to the Matrix. While the Time Lords were around, it was held by the Keeper of the Matrix.
- The Great Key of Rassilon is the key to the De-Mat Gun, which the Doctor used to lock the Medusa Cascade, which had something to do with ending the Last Great Time War, erasing Gallifrey, and time-locking the whole mess. While the Time Lords were around, it was held by the Chancellor.
- The Rod of Rassilon, also called the Great Key, raises the Eye of Harmony. While the Time Lords were around, it was held by the President.