空空 is a reduplicated form of 空 (empty). In everyday Chinese, it describes a place or container that is completely empty — nothing inside. In Buddhist philosophy, it refers to the concept of 'emptiness of emptiness' (śūnyatā of śūnyatā), a deep insight that even the notion of emptiness itself is not ultimately real. At HSK 7 you're most likely to encounter it in philosophical or literary texts.