The transwarp hub was a Borg structure hidden in a Delta Quadrant nebula, linking conduits to every quadrant of the galaxy, which Voyager destroyed in 2378 while using it to reach Earth.
The hub appears in the series finale, Endgame. Seven of Nine explains that only six exist; each connects thousands of transwarp corridors, letting the Borg send vessels almost anywhere within minutes. This one sits inside a nebula thick with cubes.
Admiral Janeway, arriving from 2404, brings armour and transphasic torpedoes so that the ship can force its way in and take a conduit home. Captain Janeway refuses to leave the hub intact, and the two devise a plan to do both.
While the admiral lets herself be assimilated and poisons the Queen, Voyager enters a corridor and fires on the interspatial manifolds holding the network together. The hub collapses behind the ship, which emerges less than a light-year from Earth inside a pursuing Borg sphere.