Antialiasing is a technique to increase image quality by eliminating jagged edges. XaoS calculates four values for each pixel (on the subpixel boundaries) and uses the average of them for the pixel value.
This filter slows XaoS down a lot and greatly increases memory requirements. It is useful mainly when you want to save images and want to make them look as nice as possible. Antialiasing also helps a lot when you want to encode JPEG or MPEG files; they are much shorter if antialiased (MPEG and JPEG hate jagged edges).
More information about filters
Available as: menu item, command line option