According to the Project Management Institute (PMI), "Project Communications includes the processes required to ensure timely and appropriate generation, collection, dissemination, storage, and ultimate disposition of project information. It provides critical links among people, ideas, and information that are necessary for success. Everyone involved in the project must be prepared to send and receive communications, and must understand how the communications in which they are involved affect the projects as a whole".
What does this mean in the real world? Keep those that need to know informed. Use face-to-face meetings, phone calls, e-mails, status reports, project status web sites, meetings, and other ways to keep users abreast of project progress, issues, and concerns.
Be aware that on many projects there will be stakeholders that are out to sabotage your project. Sometimes these actions will be obvious, and sometimes the signs will be subtle. Occasionally the saboteur doesn't even realize the damage they are doing. It could be just the way in which they go about doing their job. They may be confrontational with everyone they work with and they may not care what others think. Be prepared to gently confront these stakeholders and escalate through the management chain if necessary to keep your project's communications effective and relevant.
PMI considers Project Communications to be very important. It is one of the nine knowledge areas as defined by PMI's PMBOK (Project Management Body of Knowledge). It is an area of knowledge you must master to be a successful project manager, and if you hope to pass the Project Management Professional certification exam.