Wikipedia
Earth-Two is a fictional universe appearing in American comic book stories published by DC Comics. First appearing in The Flash #123 (1961), Earth-Two was created to explain how Silver-Age ( Earth-One) versions of characters such as the Flash could appear in stories with their Golden Age counterparts. This Earth-Two continuity includes DC Golden Age heroes, including the Justice Society of America, whose careers began at the dawn of World War II, concurrently with their first appearances in comics. Earth-Two, along with the four other surviving Earths of the DC Multiverse, were merged into one in the 1985 miniseries Crisis on Infinite Earths. However, following the events of Infinite Crisis, the Multiverse was reborn, although the subsequent Earth-Two was not the same as its pre-Crisis equivalent.
Following the events of Flashpoint, Earth-2 underwent an additional reiteration. While it still houses a team of superheroes, its membership is younger than before. Earth 2 also has a tragic backstory, having been invaded by a horde of alien invaders from Apokolips five years prior to the reboot, ahead of Darkseid's attempted invasion of Prime Earth. In the process, this reality's Superman, Batman and Wonder Woman all died, while its Supergirl and Robin were swept through a dimensional warp to Prime Earth where they became known as Power Girl and Huntress.