The index of the baked lightmap applied to this renderer.
The index refers to the LightmapSettings.lightmaps array. A value of -1 (0xFFFF) means no lightmap has been assigned, which is the default. A value of 0xFFFE is internally used for objects that have their scale in lightmap set to 0; they affect lightmaps, but don't have a lightmap assigned themselves.
The index is 16 bits internally and can't be larger than 65533 (0xFFFE).
Note: this property is only serialized when building the player. In all the other cases it's the responsibility of the Unity lightmapping system (or a custom script that brings external lightmapping data) to set it when the Scene loads or playmode is entered.
A lightmap is a texture atlas and multiple Renderers can use different portions of the same lightmap.
Additional resources: LightmapSettings class, lightmapScaleOffset property, ShaderLab properties.