Workitem is a basic workable entity on which team works on day to day basis to achieve a specific goal. It can be Story for Scrum Team, Defect for Testing Team, Campaign for Marketing Team, Lead for Sales Team.
Quickwork uses progress bar over a workitem to display progress. You can add subitems in a workitem. If all subitems status within a workitem are completed then workitem progress bar will be of 100%, and it will consider that workitem as completed.