Milestones are a way to organize work that shares a common due date. GitHub Milestones can be assigned to GitHub issues and pull requests making it easy to filter and tracking progress towards the milestone. Teams often use milestones when they're working towards a big launch date or if they're working in sprints (ex. teams that practice Scrum).
If a card is assigned to a milestone, the milestone is displayed on the card on your Waffle Board. A card can only be assigned to one milestone at a time.
For more information about milestones, check out: