I attached the description that I still not understanding 
- I checked it and changed something in the current scene view but I didnt feel like the change was the first thing to being updated.
- When and when not to use it?
Building the lighting for a scene can take hours or even days, depending on the quality settings. This setting means that the place you are looking at in the editor will be prioritized by the lightmapper, baking the light first for whatever you look at. This allows you to manually check areas of the scene and do some quality checks on whether the light looks good, and abort the bake early if something’s wrong.