If you plan a larger piece of work, it is very useful to have an outline for the work you need to do and share it with potential contributors. Your roadmap covers your goal and vision and should include a timeline for tasks that need to be completed, thus helping anyone new to your project to develop an understanding of what is currently happening on the project and what's coming next. A roadmap is also a great tool to highlight dependencies among tasks, helping you to schedule work on them efficiently.
Milestones can be really helpful to get to your main goal. Milestones can be organised around project goals, dates, events, or timeframes. If you work on GitHub, you can use GitHub's Project board to manage tasks and issues.