The Rule of Two was a Sith philosophy established by Sith Lord Darth Bane in order for the Sith to operate in secret and eventually get revenge on the Jedi Order, following their near annihilation in the Jedi-Sith War. Inspired by the concept of a Force dyad, the rule mandated that only two Sith Lords could exist at any given time: a master to represent the power of the dark side of the Force, and an apprentice to train under the master and to one day fulfill their role. The philosophy of two governed the Lords of the Sith.
The Sith carried out their vendetta for a thousand years, gaining power in secret while the Jedi still believed that the Sith had been destroyed. During this period, the Dark Lord of the Sith Darth Plagueis took Darth Sidious, the Sith persona of Senator Sheev Palpatine of Naboo, as his apprentice. They worked for years to coax the final secrets from the dark side of the Force. Sidious called back on the wisdom of Plagueis as a planner and a prophet, knowing full well that two Sith were required to enact their means; one to serve as bait for the dark side, the other to be the vessel. Success would allow them the ability to harness the full power of the dark side and rule immortal for ten thousand years.