When you are running a task that you have created (not a task received from a task owner), you will need to indicate when you have finished the task.
When the task is in progress, look in the Run section of the Task details page. This will contain a form for you to confirm that you have finished the task.
Confirm and use Finish to finish the task. The task status will be set to Complete.
If you have been sent a task from a task owner, you use Submit task instead of finish.