What is the burn and dodge tool?

The burn and dodge tool allow you to either darken or lighten an image of your choice. This can be used to create dramatic effect within an image, and so can use it to lighten dark areas or darken parts you want to stand out the most. This is used in portraiture due to how it can be used to make certain features stand out above the rest, such as making one side of the face darker.

The image below shows the effects of dodge and burn to a grey surface:
 

I wanted to apply this to an image of mine to see the effects that could be made. These were my steps:

1) Navigate to the selection bar on the left, and pick the burn tool. 

2) Select a suitable size for the brush to match the face, and go over the parts of the face I want to darken once. 

3) Go back to the bar on the left and select the dodge tool instead, from there I lightened the parts of the image I wanted to have a clear contrast from the darkness.