Metaplot refers to the timeline of major events in a given fictional universe, as published by the creator of the universe in various canon materials.
Metaplot adds detail and dynamism to the setting, and can provide in-universe explanations for rules updates or editions changes. For example, the Reckoning in Mage: The Ascension was used to justify a shift in the overall tone for Mage: The Ascension Revised Edition, and the Family Reunion of Vampire: The Masquerade 5th Edition explains the decision to unify the Giovanni and the Cappadocians into Clan Hecata.
However, metaplot does have its downsides. For one thing, it can be difficult to keep track of developments across many books and long periods of time. For another, when one's personal game or character diverges from the metaplot, it becomes harder to use subsequent supplements that assume the metaplot is in force.
The World of Darkness is well known for its expansive metaplot, which concluded with the Time of Judgment. The 20th Anniversary Editions published beginning in 2011 ignored the Time of Judgment, and made other metaplot elements optional. Vampire: the Masquerade 5th Edition also retcons the Time of Judgment, as well as some things established by V20, in order to bring the official metaplot up to the present day.
Trinity also had a more limited metaplot, both between the games and within each game line. By contrast, the Chronicles of Darkness and Exalted were explicitly designed without metaplot, though a few Exalted books offer up possible "future" scenarios, such as Return of the Scarlet Empress.