The Pixelate filter converts the image into a grid of squares. The slider determines the size of the square, ranging from 1 to 20 pixels.
15 pixel squares.
The center photo above had the Pixelate width set to 15. This converted the image into a series of blocks 15 pixels by 15 pixels. Of course, as is the case with most of the filters, the Selection tool can be used to apply the filter to only a specific section of the image. In the image on the right, only the subjects handsome face was pixelated.