In Batman vs Superman: Dawn of Justice, Batman/Bruce is having nightmares. I do not understand that, what are those nightmares all about?
And who is this guy?
Bruce had various dreams in the film and each one has its significance. Like showing his internal fears and some to highlight future events. There is very good detailed description available form looper.com as :
1. On the wings of bats
Bruce Wayne's first dream sequence happens within the first few minutes of BvS when he runs from his parents' funeral and falls into a sinkhole somewhere on the Waynes' expansive property. When he hits bottom, bats begin to swarm around him, and despite his initial terror, he quickly gets used to getting smacked in the face with leathery rabies wings. Where reality ends and the dream begins in unclear, but a tornado of bats levitates Bruce to the surface again, completely unharmed.
What it means: Jesus
Bruce Wayne, having lost all meaning and grounding in his life, discovers the unexpected thing that will give him meaning once again: chiropterology. And also heroism. Almost all retellings of Batman's origins use bats as a divine messenger that directs Bruce towards his unique brand of themed heroism, but this mythology depicts him not unlike Superman in his effortless fight, resurrected from the death of everything he held true into a new form. It's pretty Jesus-y and obvious in what it's trying to portray, but it also lends him a visual similarity to Superman that we don't often see.
2. The Wayne crypt
As Bruce Wayne stands in front of his parents' crypt, blood begins to ooze out from the sides, and after a few moments, a hideous humanoid monster, more bat than man, bursts through the stone and attacks Bruce. He wakes up in his apartment next to some unnamed woman who we never see again.
What it means: Becoming a monster
Not only is this another reminder that Batman's mom was named Martha, but the weird beast-bat shows that Bruce Wayne also fears becoming the monster that he sees in Superman. Batman has become ruthless and violent since the destruction caused by Superman, and he's begun to brand criminals, which has become a death sentence in prison. In his terrifying beast form, he's destroying himself, and he's destroying the legacy left behind by his parents. Bruce is subconsciously reminding himself to not become dehumanized, which is exactly what brings a stop to him nearly spearing Superman through the chest with kryptonite. It's also a little callback to Batman Begins' "nightmare" Batman.
3. The Knightmare
In Batman's most elaborate dream sequence, he overlooks a giant desert with crumbling buildings in the background, with the Greek letter Omega carved into the sand.
Batman and his soldiers are picking up some kryptonite, but the tides quickly turn and Batman is captured, as some of his soldiers turn out to be part of Superman's army. Insect-like alien soldiers swoop down to pick off any remaining support that Batman has, as Batman is restrained and kept in an underground bunker. Superman appears and rips off Batman's mask, but only after chopping two other captives in half with his laser eyes.
What it means: Superman the tyrant
This is the Superman that Batman fears most: a total despot of Earth who has control of everything from human soldiers to alien insects. The Omega symbol belongs to Darkseid, the main villain of the DC universe, who is the ultra-powerful alien Lex alludes to in the final moments of the film. Superman's bug-soldiers are the Parademons, who are generally controlled by Darkseid alone. How or if Superman and arch-nemesis Darkseid decide to work together, or how these symbols are related to Superman's rule of Earth, remains a mystery, but symbolically, it shows just how easy it would be for Superman to become a tyrant and abuse his powers.
4. Am I too early?
As Bruce Wayne waits for the information he swiped from Lex Luthor's servers to decrypt, he dreams that a red-clad figure reaches through a portal and starts to speak to him about Lois Lane being the key to some event, and to not trust "him." When the figure, who is very likely The Flash, sees Bruce's confusion, he remarks that he must be too early and disappears as Bruce wakes up.
What it means: Time travel
This little prophecy is probably one of the most telling in the whole film, because it sows the seeds of distrust in Batman by telling him not to trust the mysterious "him," and it also means that time travel is already a thing in the DC Universe…which has been a dangerous precedent to set ever since Superman flew backward around the world to reset time many years ago. Lois isn't the key to anything if Superman isn't around, so if the final moments of BvS didn't already convince you that Superman's death was only temporary, allow future-man to confirm that a non-dead Superman will still care about Lois in the future.
And that character is definitely Flash (Barry Allen), played by Ezra Miller. Going to be explored in future films. His costume did appear quite different, some kind of metallic one but we don't have any detailed explanation about his suit yet. But Zack did tweet a picture maybe from a costume room of Justice league and it did give a glimpse of the Flash costume in the leftmost corner
[...] I think we were in the middle of shooting and we started working on the outline for the next movie and where they go, and Zack said, ‘Oh my God let’s add this moment that is gonna pay off down the road, and we’ll find out more information.’
The first part is a nightmare about Superman becoming a tyrant and the second part is "supposedly" the flash/Barry Allen using his ability to time travel to warn Batman from a danger yet to come and urging him to gather them (The meta humans\the members of the Justice League) which Batman was trying to do at the end of the movie.
From cinemablend.com :
Bruce Wayne falls asleep while decrypting a file on Lex Luthor’s Lex OS. He has a nightmare about an Earth taken over by Superman, and then awakens to the Scarlet Speedster (The flash) screaming at him – having travelled through time to warn him of something. It’s admittedly somewhat difficult to make out what Barry Allen screams at Bruce Wayne when he shows up in the dream, but it goes something like this:
"Bruce! Listen to me right now! It’s Lois! Lois Lane! She’s the key! Am I too soon!? I'm too soon! You were right about him! You were always right about him! Fear him! Fear him and find us. You have to come find us, Bruce!"
The ominous warning is Barry Allen seemingly pleading with Batman to find Lois Lane, unite the seven members of the Justice League, and be wary of some tragedy...