The Wormhole is plot point in Run 3.
The Wormhole was first identified by the Student, but first shown in-game in the cutscene "Wormhole in Sight," after beating T-7.
After beating T-7 and discovering the Wormhole, the achievement A Glimpse of New Places is unlocked. The instructions for unlocking the achievement used to be "locate the first wormhole" (lowercase 'w') suggesting that there might have originally been more than one planned to be in Run 3. The achievement can only be unlocked by beating T-7, despite the fact that the Wormhole can be seen in the background of several other levels.
In scientific terms, wormholes are hypothetical shortcuts between two points in spacetime.
The following is a brief description of the story involving the Wormhole More information on the story can be found here.
The Wormhole is first shown in the cutscene "Wormhole in Sight," after beating T-7. The Skater complains to the Runner that they were "almost there," suggesting that T-7 ends very close to the Wormhole, but not close enough to reach it. At first, he doesn't even know what it is, only that they're trying to reach it. The Runner tells him that it is a wormhole, and that it connects to another point in the Universe. The Skater asks how they're going to reach it from T-7, but the Runner seems like she didn't plan to start from there. She tells him to "Check for any tunnels we missed," instead.
Later, in The Way Onwards (N Tunnels, I Tunnels and presumably the H-Tunnel and the "I hope I don't run out of letters" tunnel) the Runner, Skater, and Student search for a way to the Wormhole while the Angel's group is heading home and the Gentleman is making his Memory Evaluation. The Way Onwards is currently still being updated, so the group has not reached the Wormhole yet.