I think this thread exists somewhere else, but here's my take.
In RPGs, it has one of two meanings:
- The over-reaching situation that defines the players' world, even if it's not within their grasp to directly effect. It will define a lot of their world. For instance, a nuclear explosion happens in the land of the dead and suddenly it's more risky to go into and out of the Umbra.
- The over-reaching plot that players are part of, usually one that they are expected to 'solve' or directly involve themselves in. Whether this is spread out over time as a chronicle, or something as insipid and non-optional like a blizzard "weather plot".
It can be both or a combination of either. I prefer the first definition, which is what RPG authors tend to use, but we mostly see it defined as the latter.
edit: Found it. I like this kind of discussion, but if anyone wants to read up on what was said before: http://musoapbox.net/topic/1776/the-metaplot