You can absolutely create professional-level portraits using nothing but a single key light, but once you move into 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 by adding a fill light, and this excellent video tutorial will show you what it is and how it can be used to create better portraits.
Coming 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 portraiture work. Essentially, a fill light is the complement to the key light. Its job is to soften shadows created by the key light. Sometimes, whatever modifier you are using can create contrast that is a bit too much and is not particularly flattering for your subject. A fill light allows you to soften those shadows by adding some light back to them. As such, the effective power of the fill light is less than that of the key light, as the idea is to soften the shadows, not get rid of them entirely. Check out the video above for the full rundown from Verosky.
And if you really want to learn about lighting for portraits, check out "Illuminating The Face: Lighting for Headshots and Portraits With Peter Hurley!"
Das good stuff.