A Workflow is a list of users or groups of users an assignment needs to be submitted to before it is considered Approved. You can create as many Workflows as you like (although you shouldn't need to create too many) and manage them under your Settings.
Think of Workflows as a logical progression or hierarchy from one staff member or staff group to the next within a given type of story. Such as Copy Editor -> Section Editor -> Editor-in-Chief -> Adviser. You can choose to assign a default Workflow for a specific type of Assignment Type or Story Tag Group, such as a Writing Workflow for the Writing Assignment Type.