In the “dream,” he saw himself as a resistance fighter on a post-apocalyptic world that had been taken over by a rogue Superman.
The sequence was a weird interlude that ultimately pushed Batman towards taking the fight to the Man of Steel, but the originally conceived version didn’t include Parademons.
Some of the elements worked relatively well, like throwing Wonder Woman in as the tertiary hero.
Batman v Superman: Dawn of Justice was packed with a lot of material, mainly because this was the movie that was laying the groundwork for the DC Extended Universe in the coming years.