The Flash Ending and Post-Credits Scene Explained

gamespot.com:

The Flash is a lot of movie to absorb. It's got time travel and a multiverse, multiple Batmen, multiple Flashes, and a Supergirl. It's not a simple movie, in other words, and it's certainly not one that's easy to explain. So let's dive in.

Warning: This article contains major spoilers for The Flash, including all the details of its ending.

The Flash has two main modes. There's the fun and light-ish Back to the Future mode, which is most of its running time--the first 90 minutes or so. Barry, having accidentally discovered his ability to time travel when he took off running as fast as he could during an emotional moment, goes back in time and prevents his mother's death. But this has ramifications--Barry has spawned a new universe where Michael Keaton is Batman and none of his other superfriends exist.

In this world, Barry meets his unpowered younger self and his old-but-still-living mom, and then even though Superman isn't around, General Zod shows up looking for a missing Kryptonian just like he did in Man of Steel. Barry is pretty freaked out by this, since Zod is going to try to terraform the Earth and kill everyone on it. So our Barry makes sure that Young Barry gets his powers, and they go break into Wayne Manor to recruit this other Batman. And this other Bruce gives the Barrys a crash course in the laws of spacetime.

Essentially, when Barry changed his past, it not only changed everything that happened after that point--it caused the universe to correct the past as well. While the film doesn't actually say why that would be, the concept is that the universe needs linear cause-and-effect, and thus has to rewrite its own backstory to make Barry's interference with his past make sense internally.

But it doesn't