The Dodge tool and the Burn tool lighten or darken areas of the image. These tools are based on a traditional darkroom technique for regulating exposure on specific areas of a print. Photographers hold back light to lighten an area on the print (dodging) or increase the exposure to darken areas on a print (burning). The more you paint over an area with the Dodge or Burn tool, the lighter or darker it becomes.
Applying the Dodge tool or Burn tool to the background layer permanently alters the image information. To edit your images nondestructively, work on a duplicate layer. For information on duplicating layers, see Layer Basics.