According to Investopedia, Backflush Costing is a concept that is used in the production or inventory management.
“It is a product costing system generally used in a just-in-time inventory environment. Instead of recording the cost in advance, you can use backflush costing to delay the costing process until the production of goods is completed. Costs are then “flushed” back at the end of the production run and assigned to the goods. Not only does this eliminate the detailed tracking of costs throughout the production process, which is a feature of traditional costing systems, it also means that the production process can be more lean.”
In layman, it just means that the system is able to accurately calculate the cost of production for you and it does so at the end of the production run.
Read more about it here.