I don't understand what ubersampling is and how it improves graphics. Can someone please explain it to me?

It's a kind of brute-force full-screen anti-aliasing. It renders the entire scene twice at a slight offset and projects the average. It reduces jaggies and makes the game look shinier. Many find that it's even better at this than less expensive selective AA methods.

However, it takes a lot of GPU power, because you're asking what is already the most GPU-demanding game on the market to render everything twice. You shouldn't even consider using it unless you have a high-end SLI or Crossfire setup.

A good example of the effect of Ubersampling can be found at: http://solidlystated.com/software/the-witcher-2-ubersampling-comparison/

Thanks. I have an Nvidia GeForce GTX 560 Ti, which is an SLI-ready card. I don't get any slowdowns with ubersampling enabled but the GPU's fan does get quite noisy.

Hey! article at The Witcher 2 Ubersampling Comparison is a very good one.

I have read several other articles about it, but never see the difference at the screenshots.
It takes a big close-up to notice the difference between ubersampling off and on.

Thanks for sharing cjrgreen!