When all the essential steps in a task have been completed, a grey tick appears in the progress circle at the top of the task. This must be checked to mark the task as completed. This can be done when viewing the task, or on the task list.
If all of the steps in the task (both essential and optional) have been completed, the task is automatically marked as completed for you. You can change this back to uncompleted if you wish.
When the task is completed, it will be removed from the "My tasks" section, but can still be viewed in the "All tasks" section. Change the status filter to "Completed" and the task will display.