A lynchpin was a person or thing vital to an enterprise or organisation.
The Monks' 2017 invasion of Earth was stopped when the Twelfth Doctor destroyed the Monks' lynchpin. Previously, Missy claimed to have fought and defeated the Monks on a planet by pushing the Monks' lynchpin into a volcano though its unclear which incarnation of the Master did this. In Earth's case, the lynchpin was Bill Potts who overloaded the Monks' broadcast with images of a version of her mother that Bill had made up. (TV: The Lie of the Land)