The bracketing mode is for creating true HDR (High dynamic range) images.
It is basically under exposing and over exposing the image to create three different versions which when combined together will produce an image which has both the dark and the light areas exposed equally.
Unfortunately, Nokia did not include an app to combine the images on the phone. Most photo editing apps on the PC should allow you to combine and create the HDR images, Here's a free one :
Picturenaut Overview
Important: The camera has to be really steady while taking the three pictures so that they align properly. Using tripods is highly recommended.