The UV scale & offset used for a lightmap.
A lightmap is a texture atlas and multiple Renderers can use different portions of the same lightmap.
The vector's x and y refer to UV scale, while z and w refer to UV offset.
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.
Additional resources: LightmapSettings class, lightmapIndex property, ShaderLab properties.