A lynchpin is a vitally important person or an essential element. You could say that a star player is the lynchpin of the team, or that technology is the lynchpin of the California economy.
Without the lynchpin, a group or project just won't work, won't go as smoothly, or will be otherwise less successful. The lynchpin of many bands is a great lead singer, and the lynchpin of a bountiful vegetable garden is rich, healthy soil. This figurative meaning of lynchpin (which is also spelled linchpin) comes from the literal definition: a pin that keeps a wheel attached to an axle.