The storage charge is the 50–60% charge level a LiPo battery should sit at when it will not be flown for more than about two weeks. It is deliberately neither "full" nor "empty" — both extremes damage the pack.
Storing a fully charged pack accelerates cell ageing and raises the fire risk, while storing it empty risks deep discharge, after which the battery may be permanently damaged. Many chargers and battery apps have a dedicated "storage" mode that brings the pack to this level automatically.