What does that actually mean?

See https://en.wikipedia.org/wiki/Bonneville_Salt_Flats — the salt flats are extremely flat (as the name implies), and because of all the salt, no vegetation can survive. Look at the pictures: there are no trees, no grass, no hiding places at all. Anyone standing (or even lying prone) on the salt flats is visible to anyone else for miles around.

GP was saying that systems should be "transparent enough that a compromised system is obvious". I'm not entirely convinced that that's possible (On Trusting Trust should have taught us that compromised systems can create places for the compromise to hide), which means that the salt flats analogy is not a great analogy, IMHO. But at least now you understand the analogy.