Split toning refers to the process of tinting the highlights of an image in one color and the shadows in another. In the days of film, it was often applied to black and white images, but it can be applied to color images, as well.
One of the most common applications of split toning is to tint the highlights with a warm color (yellow) and apply the opposite, a cooler color (blue), to the shadows. The steps are pretty simple, but I;ve also included a downloadable preset at the end of this post that you can download to get started.
In Lightroom, open up your image of choice and head in to the Develop module.
Split toning is a combination of yellow and blue color hues. And highlights and shadows adjusting the value points from there. Typically you have to just go with what you think looks best, being a good judge of eye and tilting your screen somewhat, can show you how the picture will come out.