Everything, from rocks to humans, produce an aura. This aura consists of light energy and, with the proper technology, can be detected by other auras. Naviglobes and aircars function by detecting the auras of other things in the area.
Some people have the natural ability to see aura as light. They can then judge the character and health of individuals based on the strength and color of the light. Kalt, for example, has a very dark aura, indicating ignorance and other negative traits. Grandfather and Grandmother Three have very bright auras indicating love, happiness, and so forth. Sickness can cause an aura to dim.
Selda can tell when someone is lying by their aura.
Many Alizonians can see auras including Shere, Silfa, and Selda, but it seems that most cannot. It seems that this ability is not widely spoken of or understood because Doctor Herstand doesn't have any idea about the ability and doesn't believe Selda when she tells her about it.