The Ahch-To system was a star system consisting of two suns in a remote area of the Unknown Regions. This binary system included the planet Ahch-To, an ocean world that orbited at a considerable distance, and served as the location of the first Jedi temple.
According to Star Wars: The Last Jedi: The Visual Dictionary, the Ahch-To system is indeed a binary star system, a fact supported by the presence of twin suns visible above the planet in Star Wars: Episode VIII The Last Jedi. However, "Forces of Nature: The Wild Spaces and Wilder Creatures of the Star Wars Galaxy" featured in Star Wars Insider 216 suggests the appearance of the two suns on Ahch-To might have been a symbolic representation of the Tatoo system.