Milestones allow you to represent activities that do not require progress percentages and yet important to success of your project.
To create a Milestone, click on the hammer icon in the tool bar.
They are generally activities that are either finished or not but they can have several statuses: Not Started, Completed, Awaiting Sign-Off and Signed Off. If they are not completed by the due date they will show as delayed.
Please note that Milestones are not calculated into the percentage of actual or expected progress.