The stock level of a particular item at which you would like to replenish your inventory i.e buy more pieces of that item and stock up, is called the Reorder Point. Reorder point can be entered at the time of creating the item.
Simple select the option Do you want to notify if the item quantity drops below re-order point? to enable re-order point option while creating a new item. Provide the email address to be notified (typically you the business owner) when stock falls below reorder point, this enabling you to restock promptly when item levels fall.