Illumination and Obscurity?

The rules in the DM Guidelines PDF talk about the different kinds of illumination (bright light, shadows, and darkness), and mention "lightly obscured" and "heavily obscured," but I can't find what those states of obscurity mean in the PDFs.

Am I just missing it?

I can only assume the illuimanation has to do with conditions for being able to stealth.

"How to play" page 7 talkes about obscured areas in regards to being a condition for stealth. Page 14 talks about invisible being heavily obscured for the purposes of hiding (e.g., impossible to see).

Rogue character sheet, under Thief Hiding class feature, talks about how they can attempt to hide even if only lightly obscured.
Well in the Attack section of How to Play they talk about cover.  I'm betting it'll be the same thing.  You basically have partial cover(+2 to AC/Dex checks against attacks), and 3/4 Cover(+5 to AC/Dex checks against attacks).  Just a guess mind you.  Either that or obscured does nothing other than reference to areas where you can and cannot hide.