The milestone overview shows all the milestones you have viewing access to in the currently active project. This is determined by the groups that have been selected in the permissions tab for each milestone. If no project is active, you'll see all the milestones you have access to, grouped by project.
Navigation
At the top of the page is the nagivation where you can create a new milestone (assuming you have permission to do so), perform bulk actions if any of the milestone checkboxes are selected, and filter the shown data by searching, or by selecting a publishing state or label.
The publishing state filter will only appear if you have permission to edit, or edit the state of any milestone. Additionally, the navigation bar allows you to change the currently active project, which defines whether you get to see all milestones (if none is active), or only those that belong to the selected project.
Clicking on a milestone title will redirect you to the details page where you can see all the information about the item, including attachments and comments.
Progress
The progress bars show the percentage of completed tasks belonging to each milestones. If a milestones has no tasks, the bar remains at 0% completition.
Labels
The viewing access label, which shows the groups that have access to each milestone, is only visible to admins. Moving the mouse over the author label will show the creation date. Moving over the deadline label will show the exact date and time. Custom labels you may have selected will also be shown here.