A Communication Plan is is policy-driven road map for getting your message delivered to your stakeholders and target audience. They are required by some funding rounds to be submitted as part of the application, to demonstrate a clear plan for communicating your project and the funding bodies message to your target audience and the broader community.
A communication plan clearly defines the who, what, where and how and outlines the potential risks and the plan for managing them. They can vary in complexity depending on what is required.