From Wikipedia:
"In photography, toning is a method of changing the color of black-and-white photographs. In analog photography, toning is a chemical process carried out on silver-based photographic prints. This darkroom process can not be done with a color photograph. There is debate as to whether a toned black-and-white photograph should be considered to still be black-and-white, as simply being monochromatic is not a sufficient condition for an image to count as black-and-white. The effects of these processes can be emulated with software in digital photography."
My prior print process involved sending digital files to a service bureau that printed the images to traditional darkroom photographic paper using a very expensive commercial printer. The printer projected my digital files onto silver paper using a laser, exposing the paper which was then processed in traditional darkroom chemicals. The resulting black and white prints were about as close to neutral in tone as possible. This was generally not a bad thing, but at times I longed for the ability to control the tone of the prints.
Now that I’m printing black and white on a color archival pigment printer, the options for toning are nearly infinite. I’ve been experimenting with various shades of warm to cool, with different proportions in the shadows and highlights. The image on the left is a neutral black and white conversion with zero toning. The same image on the right has a subtle warm tone applied only to the shadows, with the highlights left completely neutral (this is called "split toning"). It’s interesting to see how the addition of a little warmth in the shadows increases the impression of depth in the photo, while also subtly altering the mood of the image.