Subtasks and sub sub tasks allow you to create a task hierarchy in your workflow.
To change your tasks to sub or sub sub tasks, all you need to do is find your project.
Then in the Tasks section, click into 'Add/Edit Tasks'.
Make sure to add a task first if you haven't already.
Then you'll see two arrows on the left of the task title.
Simply click on the right arrow to change it to a subtask, or click it on again to change it to a sub subtask.