Milestones - slander36/IssueTrackerTraining GitHub Wiki
Milestones are key delivery dates for the project. If you have a company wide test scheduled, that's a milestone. If you have a demo to investors, that's a milestone. If you have a conference you're showing it off at, that's a milestone. And, of course, launch is a milestone.
Milestones are important for keeping track of what stage your project is at, and should be treated like mini-launches. Code locks are good things to have in place, meaning when you are 1-2 weeks out, no new features go in and any that are still being implemented are moved to a future milestone (unless utterly required for this milestone). When a code lock is in place, the team creates a new fork of the project in which the only changes that can be made are issue fixes, and even then only if tested and approved.
Other parts of the team may continue development, but only on a development branch. Once the issues are fixed, the milestone code is merged back into development and life continues as per normal.