The file is supposed to be a list of all of the meta-humans that Waller knows of.
Bruce Wayne already knows about the existence of 3 meta-humans from the files he stole from Luthor in Dawn of Justice (Cyborg, Flash, and Aquaman), but he assumes that Waller knows more. So, he somehow convinces or coerces her into handing over her files on those people, and any others. Note that the second dossier of Arthur Curry, which is likely where Bruce learns how to track him down.
This scene is leading up to the Justice League movie, where Bruce tracks down these metas and convinces them to join him to fight crime. His last words to Waller are warning her that her attempts to use supervillains for her dirty work will put them in conflict with himself and his group of superheros.