On program, aperture priority, or shutter priority mode you aren't controlling all 3 aspects of exposure (ISO, aperture, and shutter speed) so the camera picks some of them for you (in program it picks aperture and shutter, aperture picks shutter, shutter picks aperture, and ISO is picked by you or the camera depending on ISO Auto being on or off). So the camera decides "this is the correct exposure at 1/60 and f/5.6". But you might say "actually camera, you are being fooled a bit, can you underexpose by 1 stop?" and to do that you'd dial in an exposure bias of -1. Then the camera would pick 1/120 and f/5.6 instead. - Benjamin Golub

So exposure bias is a way of controlling what the "correct" exposure is. - Benjamin Golub

A good way to see this happen is to flip your camera to program (P) mode, look through the viewfinder, and watch the shutter/aperture change as you dial in different exposure bias. - Benjamin Golub

Is there much benefit to exp. compensation when shooting in Manual? I shoot manually and have never fiddled w/ exp. compensation but on one or two occasions, just to see if any change occurred. Did not notice anything so have not returned to it. - JA Castillo

No, there's no benefit to it in manual mode. In manual mode, you just look at the meter and underexpose or overexpose to do the same thing. - Wirehead

Ah, so it's more a function for those modes outside of Manual. Very well...thank you! - JA Castillo

The easiest way to think of it is "Do you want your pictures lighter or darker?" for anything other than manual mode. - Brian Johns