Root cells kick off all the action. They're input items--the places where you and your audience can change data values.
These are cells whose values are driven by other cells. They are not editable by people who you share your plan with, so it's impossible for your audience to break your model.
Terminal cells don't control anything. They might be "the bottom line" in your model, or some other kind of summary value.
As an auditing tool, their value becomes immediately apparent when you see one in an unexpected place. Maybe in the process of building your model you forgot to connect a particular input, or you broke a formula in the course of making some changes.