I don't understand that. What is this?!

Resolution for the following meshes was reduced to fit each mesh into a lightmap:
Group reduced to 58
Resolution for all meshes was scaled down by 19% to fit the scene into 2 lightmaps.
다음 메시의 해상도는 각 메시를 라이트맵에 맞추기 위해 감소되었습니다.
그룹이 58개로 축소됨

What is that mean?
And why reduce my meshes?

You can find an explanation here: Lightmap resolution related changes