To understand the concept of meta-regions, it helps to know how modules were distributed at the beginning of the LG campaign.
There were core and regional modules, as Mr. Pleasington pointed out, but there also modules called adaptables, written with a broad focus that the campaign presumed could be applied to every region. The depth of the regions became very apparent when adaptables couldn't be adapted to all of the regions, because of the inclusion of certain things that were just not possible in that area (whether it be the presence of devils/demons, nobility, particular faiths, etc).
A more practical solution was to group the various regions into "meta-regions" based on their Greyhawk geography. Regions that bordered the Empire of Iuz, that bordered the Kingdom of Nyrond, the Sheldomar Valley, the Velverdyva/Baklunish West trade route, Sunndi were all combined into meta-regions so that stories could be created that would be a more practical fit for those locations.
Whereas someone from the Dyvers region can't play Verbobonc modules without going to that region (Illinois or Indiana) and similarly someone from Verbobonc couldn't play a Dyvers event without going there (Missouri, Kansas, Nebraska, Iowa), both regions are in the same meta-region, and thus a meta-regional event could be played in either region.
Think of it as the training circles from Mask of Zorro. Regional events are very specific, they can only be played in one region. Meta-regionals can be played anywhere in the six regions that compose it. And cores can be played anywhere.
