A light map stores lighting formation for an object via a texture. Light map UVs have to be unique, and not overlap, but UE4 will generate one for you based off your existing UVs. But you can provide your own lightmap UVs if you wish, normally it’s mostly useful for meshes like a floor where most of the lighting information is on one side of the mesh. If you are using completely dynamic lighting, you don’t need lightmaps or UVs for it. But baked lighting is typically better looking and has minimal performance impact, as long as you can deal with render times and it being more static.