A transwarp aperture is an opening to a transwarp hub. The aperture is the center point of a wormhole, it's most vulnurable spot.
In 2378 the USS Voyager entered aperture 823 of a hub in the Delta Quadrant. It entered corridor 09 which is the transwarp conduit that has an exit aperture with one light-year Earth. Upon entering the conduit, the ship launched a series of transphasic torpedoes back at the supporting interspatial manifold. The simultaneous attack on Unimatrix Zero caused the whole transwarp hub to collapse. The Borg were destroyed in that same attack. (VOY: "Endgame")