You can absolutely create pro-level portraits using just one key light, but once you move up to multi-light setups you get a bit more creative and technical control over the final image. One of the first ways to do this is to add fill light, and this excellent video tutorial will show you what it is and how it can be used to create better portraits.
Come to you from Ed Verosky, this awesome video tutorial will show you the ins and outs of fill lights and how to use them in your portrait work. Essentially, an accent light is the complement of the main light. Its job is to soften the shadows created by the main light. Sometimes, whatever modifier you use, it can create a little too much contrast that isn’t particularly flattering for your subject. A fill light allows you to soften these shadows by adding a little light to them. As such, the effective power of the fill light is lower than that of the main light, because the idea is to soften shadows, not get rid of them completely. Watch the video above for Verosky’s full recap.
And if you really want to learn more about lighting for portraits, check out “Illuminating the Face: Lighting for Portraits and Portraits with Peter Hurley!”