Little is recorded of Hallas after he succeeded his father as Steward. He ruled Gondor in the time when Rohan was establishing itself - the Golden Hall of Meduseld was completed in the second year of his rule. At the great feast to celebrate the raising of the Hall, KingBrego's son Baldor made his rash promise to enter the Paths of the Dead, which led to his loss in the mountains. It is even possible that Hallas was present in person at that feast, though no record exists to prove this.
The date of Hallas' birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
Sindarinhall- meant 'high' or 'exalted', and that is almost sure to be the primary element of Hallas' name. The final -as is less clear, though that form was often used as a noun-forming suffix, so the simplest interpretation of the name Hallas would simply be 'highness'.