As Vikki explained the tool lightens and darkens areas ..but they can be used on other things than photos!
Its sort of hard to explain. I use the burn tool to paint in impressions like for example when i create a strap like the in the image attached. I have used teh burn tool to paint in impressions from the metal bar underneath, the make it look as if perhaps teh inside edges of the strap and the very outside ends our folded over a bit an dthat the brads are pushed into the serface. I did this example pretty quick but hopefully you will get teh idea

You can also use it to enhance shadows or any number of things really
