Milestones are collections of Epics, which in turn are collections of Stories. Similar to an Epic's Burndown Chart, the Milestone Burndown Chart helps track the remaining Stories across all Epics that are part of any given Milestone.
Milestone Burndown Charts are located in the "Reports" tab that can be accessed by clicking into a Milestone. The Milestone Burndown Chart will only reflect associated Epics that are in an In Progress or Done state:
Reading the Milestone Burndown Chart
The x-axis of the Milestone Burndown Chart represents time, while the y-axis measures number of Stories within the Milestone. The x-axis will start at the date you defined as the start date for your Milestone.
The number of outstanding Stories in a Milestone Burndown Chart is visualized with a blue line, while the orange line represents the projected burn of work within the Milestone. Since not all Milestones necessarily have set End Dates, Burndown Charts can help predict the date when all of the Stories in a particular Milestone will be completed, based on your current burn rate.
Hovering over any point on the Burndown Chart will show you the current and projected progress at that point in time.