I agree with @Raistlin’s definition of metaplot as an overarching story that impacts every character on the game.
My example from The Savage Skies would be “The Global Fight Against Fascism.” So metaplot involved trying to protect an area of northern Spain during the Spanish Civil War, softening the landing of Czechoslovakia into the Drachenordenung orbit, collecting magic spheres to raise Atlantis, and preventing the Hollow Earth from becoming a fascist garden were all chapters in the metaplot.
On the other side, exploring ruins for magical artifacts, meeting with dragons, random skirmishes with fascists, and the like weren’t necessarily part of the metaplot, but sat within it, defined by it.
On The Fifth World, the metaplot was “The War Against the Hostiles.” All the noble political maneuvering and concert series and Senate elections and the like were smaller plots that were placed in context by the metaplot.
At the simplest level, I would say that Metaplot is the stuff that NEEDS to have a post on the game boards, simply so that everyone knows what’s going on with it.