Order processing consists of a chain of actions over an order. An order is assigned such a specific property as status used to define the current stage of order processing. An order status changes from action to action.
An order status change is called transition. A transition can be performed automatically, depending on an action result or manually, this is up to a transition's settings.
Right after creation, an order gets some initial status. Normally, the initial order status is New (the default status meaning is that manual approval is needed to allow further processing) or Open (the default status meaning is that an order is ready for further processing). An order goes through various actions during processing and depending on actions' results changes its status. Successfully processed order typically gets the Completed status.