By now you may have noticed the "Preview" labels in the shadows and the light leaking under walls.
This is because all the lights in the scene are static and use precomputed, or baked lighting, which has not been calculated yet. The "Preview" text is there to remind you that what you are seeing in the viewport currently is not what you will see in the game.
In this step we will go through the Build process which will build all levels (precompute lighting data and visibility data, generate any navigation networks and update all brush models). We will also take a look at Light Quality settings inside of the Build Options which we can use to adjust the quality of our lighting when it is built.