means The central wedge-shaped stone at the top of an arch that locks all the other stones in place, and by extension the one crucial element that holds a whole system together.

from A plain English compound of 'key'—in the old sense of something that fastens or locks—and 'stone.' In masonry the topmost wedge does exactly that: it jams the two halves of an arch against each other so nothing can slip. From the literal stone the word spread outward to mean any indispensable part, the piece you dare not remove.