In "Batman v Superman: Dawn of Justice," Bruce Wayne experienced an extended dream sequence that showed a desolate future where Superman was treated like a god and destruction reigned. Immediately afterwards, Batman got a visit from the Flash, who had a cryptic message for Bruce before he disappeared. During an appearance on Empire's podcast, director Zack Snyder provided a little insight into the sequence and why it is important to the DC Cinematic Universe.
"I think it's okay to look at the extended dream sequence as an impressionistic view of a possible future," he explained. "That's... in the sequence, so I'm not like spoiling anything or making up anything that you didn't see. So the connection with the Flash... part of that sequence, you can speculate if, whether or not Flash is coming from that reality or another one, but that's the fun stuff to try and figure out exactly what Flash is saying to Bruce and what he means... we know, so... we're not making it up."
As to Darkseid's connection to the Knightmare, he teased, "Oh, that giant impression in the ground? Oh yeah, that's right. I mean, maybe [Darkseid] exists out in the universe somewhere, just looking for something, something that's against life, I don't know."