The Flash has always been the key to the DC Multiverse

gamesradar.com:

The concept of the Multiverse, an interconnected web of different realities where all possibilities are realized across countless timelines, is having a big pop culture moment - and not just in superhero media, where it's become the current focus of numerous stories.

That includes The Flash, the new DC film in which Barry Allen teams up with a version of himself from another world in a story that crosses the DC Multiverse and brings in alt-versions of heroes including Batman and Supergirl.

It may seem like The Flash is tapping into a modern pop culture zeitgeist, but the Flash himself has connections to the DC Multiverse that go way beyond 2011's reality-rebooting Flashpoint comic event which provides inspiration for the movie, all the way to the very roots of the DC Multiverse in the Silver Age of the '60s.

In fact, the DC Multiverse itself first appeared in 1961's The Flash #123 in a story titled 'Flash of Two Worlds.'

In that story, Barry Allen uses a strange aspect of his super speed abilities to vibrate the molecules of his body so fast he seems to disappear. But rather than simply reappearing a moment later, he is transported to Earth-Two (which got its name later on), where he meets not another version of himself, but instead the Golden Age Flash, Jay Garrick, whose adventures had been published in the '40s.

This established the longstanding tradition that Earth-Two of DC's Multiverse is the home to the Justice Society and its members, many of whom are the original Golden Age versions of DC heroes such as Green Lantern, Wonder Woman, and of course the Flash.

After that initial story, Barry Allen wound up crossing between worlds numerous times in a series of annual meetings between the Justice League of Earth-One