The highlight controls appear on the main Highlight tab. They control how shiny the material appears.
The graph is a 2D representation of how the highlight will appear (if there is a highlight, a miniature graph appears in the tab itself). As you increase the Level amount, the curve grows taller, and as you increase the Glossiness amount, the curve grows narrower.
Level: Affects the intensity of the specular highlight. As you increase the value, the highlight grows brighter. Default=0.
Glossiness: Affects the size of the specular highlight. As you increase the value, the highlight gets smaller and the material appears shinier. Default=10.
Soften: Softens the effect of specular highlights. When the Level is high and Glossiness is low, you can get harsh backlights on surfaces. Increase the value of Soften to mitigate this effect. At 0, there is no softening. At 1.0, the maximum amount of softening is applied. Default=0.1.