A metaplot is an ongoing story taking place in a game world, told in installments via the supplements provided for a particular RPG. For example, the corebook of an RPG may mention that a particular group of people are investigating a mystery; a later supplement may describe what they found, the impact that the find had on the world, and offer rules representing that impact. As another example, the corebook may list a particular powerful wizard as the leader of a particular faction: a later supplement may announce that that person has defected, and then describe the faction in detail without the benefit of their leadership, possibly including loss of magical abilities as a result.
Metaplots are a favoured method used by RPG publishers, in particular White Wolf, to encourage the purchase of supplements. Many gamers enjoy the excitement of a continuously evolving game world, and the slow revelation of mysteries within the setting in a manner similar to serial fiction.
However, many players and GMs dislike metaplots. The most common criticism is that play groups are forced to follow the metaplot by the threat of losing product support if they don't. In the above example, if the results of a group's adventures logically mean that the wizard in their gameworld will not in fact defect from the faction, they will be left on their own to work out how the group's magical abilities would have developed, because no supplement will ever be provided covering this. If however the GM insists on forcing events to conform to the metaplot by declaritng that the party's actions had no real effect and the wizard defects anyway, then the PCs have been railroaded and deprotagonized, potentially creating dissatisfaction.