LS had some rather spiffing explanations for what AA actually is and how it works, but in short, it's basically the smoothing off of sharp edges in graphical pictures. Turning it on makes a
huge difference, which is usually for the worse as it brings frame rates down like a ton of bricks. To turn it off, check out your gfx card options in Control Panel -> Display -> Settings, and you should be able to turn AA off in there. It's not worth it really.
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