Milestones can be goals, versions, sprints or whatever you like really. They are a collection of tasks that you can create and manage in Project Settings > Milestones
Make a new milestone and set a name and due date. Once you create some milestones you will be able to add tasks to them from inside a task and also when you multiple select some tasks.
Close a milestone when all the tasks in it are ready. You can also filter tasks by milestones and copy milestone release notes which are basically just all the tasks inside a milestone.