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.
Milestones do not contribute to the Project Completion Percent.