Pick at least one mesh before running this tool. It generates a second UV set named “LightmapUV”. This new UV set has no overlapping UVs and is for correct lightmapping on static meshes in Unreal. It’s a convenient tool for a quick fix. You may unwrap the UVs manually for better lightmapping results. For more details, please refer to UE4 document: Unwrapping UVs for Lightmaps.