Milestones are great for keeping track of important dates or project updates. From the people view of the schedule they appear as colored dots that can be hovered over to reveal more information, and from the project view they display across from the project name on their assigned dates.
You can edit existing milestones directly on the schedule, and they can be added from multiple places:
From within a project card on the Projects page: To add a new milestone, select the Milestones tab from within the project menu, enter a name and date for it, and then click Add milestone.
From the Projects view on the Schedule: Click the date you want to place the milestone to bring up the Milestones menu.