Earth-S is one of many Parallel-Worlds that exist in the Multiverse of possible realities. It can only by accessed by circling in flight a specific number of times around the mystic Rock of Eternity where the wizard Shazam resides (first appearance: Whiz Comics No. 2, Feb 1940: "Introducing Captain Marvel").
Earth-S is the version of Earth that is home to the Marvel Family and Shazam's Squadrom of Justice which consists of the heroes Ibis, Spy Smasher, Bulletgirl, Bulletman, Mr. Scarlet, and Pinky (Justice League of America No. 135, Oct 1976: "Crisis in Eternity!").