The Veil

The Veil, or the Furthest Ring, is a large ring of meteors in the Incipisphere that forms the outer boundary of The Medium. Near the end of a Sburb game session, the Black Queen and King cause the Veil to fall towards Skaia in an attempt to destroy it, in a phase of the game called The Reckoning. At this point, the King and Queen must be slain before the meteors destroy Skaia. Skaia buys time by catching some of the meteors in portals, though it cannot fend them off for long.

Apparently the meteors contain buildings like labs, factories, and facilities for genetic engineering. The Veil is considered a neutral ground between the two kingdoms, and is where the Trolls are when they troll the protagonists.

It is heavily implied that The Veil in general, and The Reckoning in particular, are in some way connected to the meteoric apocalypse on Earth. gallowsCalibrator has also referred to the apocalypse on Earth as a "Reckoning" in her first conversation with Rose.

In addition, Rose's Grimoire mentions the veil by its other name, the Furthest Ring; it states that the most fearsome monsters of the Noble Circle of Horrorterrors may be found there.

Theory
It's possible that the Reckoning had already begun before the story began. The meteors seen flying into Earth are summoned from portals that closely resemble the Seven Gates; carcinoGeneticist said that Skaia would defend itself from meteors with portals, so perhaps the meteors striking Earth are the redirected attacks on Skaia. If this is true, it would mean that the Homestuck kids' Sburb session had to have begun long before they began playing, though this would explain why their guardians are so aware of the game and there are so many documented meteor attacks in the past. carcinoGeneticist also said that the Homestuck kids had an abnormally small window of time to complete the game, and that they'd made a mistake a long time ago, which would give this theory even more validity.

If this is true, then it's also possible that the various technologies that grew from the meteor craters are connected to the original labs and factories on the Veil meteors.

Given that the meteors on Earth are scattered out over such a large stretch of time (One of them, seen in Jade's vision, happened millions of years ago, another presumably carried John's Sassacre text back to earth a long time ago, and another presumably seeded Skaianet Labs), it is just as likely that the portals send the meteors to random points in time.