Be able to otherwise

[Maybo]

Even though it's dark in the room, it's lighter than it was in the hallway, so our eyes are already adjusted. Which means I'm able to make out details I wouldn't be able to otherwise.

Source: The Inmate by Freida McFadden

What does the underlined part mean?

 

[emsr2d2]

Bear in mind that established writers can write what they like, but we would always discourage a learner from starting a sentence with "which". Also, for info, BrE would say "our eyes have already adjusted".
Anyway, the fact that their eyes have adjusted means they can discern details of the room that they wouldn't be able to discern if their eyes hadn't adjusted to the light level.