A Keystone is the block in the top center of a stone arch. If removed the whole structure will collapse.
Watch:
https://www.youtube.com/watch?v=BGIN8dgesM4
POTUS and the US intel apparatus were the center block (keystone) of Cabal structure. Now that they are removed, the structure is doomed to collapse.
That was my interpretation, anyway. But I admit that it sounded like Q was talking about something more, a specific piece of the puzzle that we haven't found yet.