Some criticize Batman v Superman for its complicated story line. There were a lot of things thrown into the movie from a combination of various comic book series. One of the most obvious influences on the movie is Injustice: Gods Among Us.
The Flash popped into Bruce Wayne’s reality for a quick few seconds, but what he said wasn’t clear and his whole purpose in showing up at that point was a bit confusing. But the comics hold the secrets. The film’s producers were layering one comic book story line on top of others, and all the audience could hope for was that the next few DC movies would make sense of the chaos.
But this video explains exactly what the Flash said to Bruce Wayne and why he appeared in the movie, helping to straighten out many of the plot points and the reason for the main conflict. If all this is accurate, then the Flash holds the key to understanding the movie as a whole.