Essentially, split toning is adding at least 2 colours to a (traditionally) black and white photograph. One to the shadows area and another colour to the highlights area. Usually the shadows get a cooler colour than the highlights area, which usually get a warmer colour.

Unlike single toning or e.g. sepia, the added interest of working with split toning is that, as there are always at least two colors added, contrast is accentuated as well as giving a focal point or a symbolic meaning to the image.