Editing is one of the final steps in the mastering process. It can range from the simple, (like cleaning up count ins) to the sublime (like restructuring the form of a song to make it more radio friendly). Usually it falls somewhere between the two. Basically with editing, you are taking your final processed stereo file and cleaning it up in preparation for the final step in the process which is sequencing and burning the master disc. During the editing process, all of the noise prior to the start of the song is removed, the song endings are faded within the editing program to clean up any human error from the mixing process (evening out the "fade envelope" with computer precision). Editing is also the stage where you can address problems like removing a chorus to make a song more radio friendly, copying and repeating a bridge section to lengthen a piece or even replacing the intro from one take with the intro of a different take.
