Demand planning as a process is taking data from multiple sources like sales or order history, current back-ordered stock, future promotions, external channel demand, etc. Then using said information to try and predict what inventory demand would be in the future.
While it’s not a perfect science and sometimes unexpected events can throw a wrench in your forecast. Typically the more data points you can feed your model the more accurate it will be over time.
Advanced order management and warehouse management systems like Shipedge store many of these data points and could even integrate with external data sources to improve the forecasting process.