Image after burn and dodge
During the days of film photography, a photographer could make specific portions of their print brighter or darker through a process called dodging and burning. By covering parts of the negative when the photograph paper was being exposed the exposure on the remaining portion was increased. Dodging was the term used to refer to trying to lighten or increase exposure on a certain area. Burning was used to decrease the exposure or darken certain areas of the print.
Most photo-editing software has tools labeled Dodge and Burn that perform similar functions and GIMP is no exception. The problem I’ve found with using these tools is that once you use them and close your image, the changes are permanent. Occasionally I’ve wanted to go back and alter those changes later on, so I would have to find the original image and work from there. The problem with this approach though is that you have to keep two copies of the image, the original and the altered one.
One of the buzzwords around imaging editing that has gained prominence over the past couple of years is non-destructive. The idea is that you don’t alter the original image, but instead make changes via other methods. This allows you to maintain the original image for use in other ways. But if you feel like using the Dodge/Burn tool destructively, you are free to do that too. 
We’ll start with this image I took in Las Vegas a few years ago. While for the most part I was happy with the original, there are some hot spots and some spots that are dark where I lost some detail. Dodging and burning can help with that.
Original image
1. In GIMP, click on the foreground color box and set it to grey using an HTML notation of 808080.
2. Create a new layer by clicking Layer in the menu bar and choosing New Layer. In the New Layer window, check the option for Fill Type of Foreground color
3. Set the new layer to Overlay mode in the Layer panel
4. On the new layer, paint with a black brush to darken selected parts of your image (dodge it) and paint with a white brush to lighten selected areas (burn it)
5. Turn off the original layer, to see the dodge and burn layer so see what you have done.
6. I usually check this layer to see if I missed areas where I want to make adjustments, and fill them in.
7. Once you are happy, go ahead and save you image. If you decide later to make more changes to your dodge and burn, you just need to go back to that layer and repaint.