A Roadmap is a business planning technique which lays out the short and long term goals of a particular project within a flexible estimated timeline. For an emerging product or a startup, the roadmap should articulate the goals and vision of the project, while laying out the development milestones with a rough time estimate for achieving these milestones.
Internally, the roadmap is used as a strategic vision for focussing the development team and providing clear goals and milestones for shipping a completed product. It also gives a sense of priority to specific tasks based upon the intended structure of the finished product. For investors, the roadmap provides insight into the direction and vision for the project, while also serving as a measure for the pace and success of the evolving product.