It is a unique substance made of a stellar core. One piece of it encloses tons of energy, which makes it a really valuable resource. We use it for everything: restoring our starships, building The Gates, hiring Nofts, and deciphering the Drawings.
Astron discovery brought about a time referred to as the Dawn of Nofts Civilization. We still keep finding it in the ruins of extinct civilizations, the remnants of stellar explosions, and near black holes. Few pieces were found on some planets and asteroids. And you can also loot it. Astron is the most precious commodity at our disposal, so we generally use it as a form of currency.