Deadline dates are handy way to alert you to problems in your schedule, but they are often overlooked. Project managers will often try to create a deadline for a task by setting a task's start date or finish date. But entering the start and finish date sets a date constraint (or restriction) on the task, which limits the flexibility of your schedule and prevents Project from automatically handling the scheduling of your project.
If you want a task to start or finish on a certain date but want to retain the flexibility of the schedule, you can enter a deadline date. Double-click on a task, and on the Advanced tab, enter a date in the Deadline box. A green arrow will appear on your Gantt chart that marks the deadline date. And if the task doesn't finish by the deadline date, a nasty icon will appear in the Indicators column to warn you.