The Spot Removal tool in Camera Raw lets you repair a selected area of an image by sampling from a different area of the same image. Its default behavior enables you to mark areas to touch up by dragging the brush across the photo. For example, in the following photo you could remove a portion of the wire (connecting the helmet and the overhead wire) that is distracting the view of the blue sky.
Using the Spot Removal tool on a raw image means that you are processing the raw image data directly. Working with raw image data directly can provide cleaner matches for retouching (healing or cloning) actions. Also, since any edits and modifications to camera raw images are stored in sidecar files, this process is non-destructive.