Lightmap is a term used for a lighting concept, where all the light data is saved in maps and the lights themselves are disabled at runtime. This means that there will not be a real-time lighting for those baked objects. This is widely used in games in order to reserve GPU calculations for something else.

The process of building a lightmap (lightmass) itself is not complex; it is just a case of hitting the Build button. But what takes some time is the tweaking done before the button is pressed.

The rule of lightmaps is that only static objects can be baked into lightmaps, which means any movable item at the level will not be written to lightmaps, and that includes, but is not limited to, characters and enemies. This means that you have to make sure that the Mobility setting inside the Details panel for all the items that are not meant to move in your scene is set to Static.

That takes us half-way to lightmaps. The other important setting is the resolution of the...