The Portals were guardians of the Tkon Empire that confronted all those seeking entry to the Empire and decided which petitions for entry would be granted. Portals were invested with tremendous power, including the ability to immobilize starships and manifest themselves in various corporeal forms according to the appearance of the petitioners. The Portals also had some form of telepathy.
One such portal was discovered in 2364 to have survived the 600,000 years since the fall of the Tkon Empire. Termed Portal 63, he was encountered by the crew of the USS Enterprise-D in the Delphi Ardu system. (TNG: "The Last Outpost")