The nanosuit thingy says this whenever I'm approaching potential hell. What does this mean? Do I press something for it to give me details?
What this means is that you can bring up your visor (I think the default is "B", not sure though, haven't played in a while) and you'll be able to see yellow squares with letters inside of them.
If you aim at these squares, they'll have different information (some examples include "Resupply", "Stealth", "LedgeGrab"). They generally reveal hidden weapons chaches, ammo caches or alternate routes.