Light Probes

When you bake light into a scene the light bake determines how much light or shadow touches each surface in the scene, and the relationships about how light bounces through the scene. Because dynamic objects (like DOM objects or avatars) aren't present at the time of bake there is no information for how they should receive ambient light.

The Light Probe's role is to tell these dynamic objects how they should be lit from ambient light sources that are baked into the scene. As a result a good habit is to place probes in and out of areas with drastically different lighting.

A more in-depth explanation about Light Probes can be found in Unity's Official Documentation.