The reverse or back side of something; the hidden side or what is behind the scenes. Can also metaphorically mean the underlying circumstances.
このシャツは裏側にタグが付いている。
This shirt has a tag on the back side.
プロジェクト成功の裏側には多くの苦労があった。
There were many hardships behind the project's success.
模型の裏側をテーブルの上に向けて置いた。
I placed the model with its back side facing up on the table.