*Warning, Spoilers Ahead* So, the whole cause of the DC Universe reset was due to the Flash trying to go back in time to save his mother from certain death at the hands of the Reverse Flash, only to cause time to shatter resulting in the events detailed in Flashpoint (and its accompanying books). Upon realising this, The Flash goes back in time again in Flashpoint #5 to stop himself from going back in time to save his mother, and this results in the new reboot of the DC Universe, due to the merging of timelines.
Justice League #1 is the first book to carry the new DC reboot. The character of Superman, Batman, Green Lantern and even The Flash have received superficial changes but for anything deeper, one will need to go through the upcoming books for the respective characters. I particularly like the Superman and Batman costumes, Superman seems to have ditched the spandex look, for a more armor-like suit. And Batman, being the only superhero with ZERO superpowers (except being extremely bad-assed), packs even more armor and surprisingly even shoots a grappling hook THROUGH an alien adversary.
Most certainly everyone looks younger here, mainly due to the fact that the book is set 5 years in the past of the new DC Universe reboot. Haha. Confusing, no? I can’t wait for the next issue though, it’s gonna be the newly rebooted Superman, versus the newly rebooted Batman! Yeah!