Milestones help track the progress of a goal over time. When creating a goal, you can add in as many milestones as you choose along with deadlines for when the milestone should be completed. By clicking the MORE DETAILS button next to a goal you can see the milestones for that goal and which ones have been completed so far.
Mark a check next to the milestone when completed and the progress bar will reflect accordingly. In the comment text box, you can describe any further details or accomplishments about the each specific milestone.