This page describes the behavior of a Light component when you set its Mode property to Baked. These are also known as Baked Lights.
Unity performs the calculations for Baked Lights in the Unity Editor, and saves the results to disk as lighting data. This process is called baking. At runtime, Unity loads the baked lighting data, and uses it to light the Scene. Because the complex calculations are performed in advance, Baked Lights reduce shading cost at runtime, and reduce the rendering cost of shadows.
Baked Lights are useful for lighting things that won’t change at runtime, such as scenery.
Note that if you disable Baked Global Illumination in your Scene, Unity forces Baked Lights to behave as though you set their Mode to Realtime. When this happens, Unity displays a warning on the Light component Inspector.