When interstellar Airts form a strong enough connection between one star system and another, it becomes possible for a Star-Pilot in contact with a starfaring engine to fold space. The link between the two is called a wormhole. Wormholes make interstellar travel possible.
Portals are the openings to wormholes. Unlike Airts, they are visible to the naked eye within a few light-minutes by the warping of background space and their interior illumination. Entering portals is a difficult process of navigating deep whirlpool Gravity Wells, so most captains prefer to have a seasoned Star-Pilot at the helm when they make the attempt.