You should see some very faint red lines marking off squares about 1/4 mile on a side. Anywhere within a particular square you will see the same reading. I suspect this number represents the amount of light being emitted from within the square as a whole. The square to the north of me has more trees and a lower number. The square to the east of me has more houses, fewer trees and a higher number. A square 1/2 mile to the west has some commercial areas and an even higher number.
For astronomy, we want fewer trees and fewer buildings. Except our own, of course. Those are OK.