[Larne]

In the "old days" one printed images using light shining onto photographic paper. The more light that fell onto the paper the darker the image would become.

So, if you wanted to make part of an image darker you would 'burn' it - allowing more light to fall onto it, usually through a hole in a piece of card which you would keep moving slightly to avoid hard edges around the target area.

Alternatively, if you wanted to make part of an image lighter you would reduce the amount of light falling onto it. This was done by interrupting the light over the target area, usually with something the shape of a lolipop on a long stick. Again you keep it moving to stop the stick causing a shadow and to avoid hard edges. This was called 'dodging'.

So, to summarise - burning = make part of the picture darker, dodging = make part of the picture lighter.

These days it is all done electronically with Photoshop having dodge and burn tools.