Originally Posted by Zenicetus
It's an effect that adds a "glow" on highlights in the graphics, if your video card has the hardware to support it. Try turning it off, then back on, and you'll see the effect.
I can't stand it myself... it looks like I'm viewing the scene through a dirty camera lens. It's an over-used effect in many modern computer games. I think some developers use it just because it's available on the card, and not for any real aesthetic reason. At least it can be disabled in M2TW, which I appreciate. I know some people like it.