Lightmap Resolution allows you to control the detail of the baked lighting that static lights produce.
On Static Mesh Components, lightmap resolution is set either on the Static Mesh asset, or by checking the Override Lightmap Res checkbox and setting the value. Larger values mean higher resolution, but also longer build times and more memory.
On Brush surfaces, lightmap resolution is set via the Lightmap Resolution property. This is really setting a density, so lower values produce higher resolution.