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 particular 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 players and GMs also enjoy the excitement of a continuously evolving game world, and the revelation of mysteries within the setting.
However, many players and GMs also criticise 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. For example, in the above example, if the GM decides that the wizard in their gameworld will not in fact defect from the group, 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. In turn, the result of this is that PCs become deprotagonized or railroaded because the GM cannot risk them taking action that would disrupt the metaplot. For example, the players may declare war on the faction to which the GM knows the wizard will defect at a later date: if they successfully destroyed it, the metaplot could not proceed and the game would be left unsupported, and thus the GM must ensure that they do not do so.