Rob Bernhard , Mar 26, 2008; 10:07 p.m.
+2 and -2 (and values in between) are called Exposure Compensation. They are there for you to tell the camera that for a particular scene you want it either brighter (zero to +2) or darker (zero to -2) than what the camera /thinks/ it should be.
This has nothing to do with being outside or inside or cloudy or sunny. It's just an available option for you to override what the camera thinks your exposure should be.