A content management system is a tool or set of tools to allow business owners or their employees to update web site content (and some times even web site design) using simple tools with little or no need for extensive knowledge of HTML, CSS, and other internet technologies and code.
Having content management tools for your web site means that you can change some or all of the web site as your business needs change, without having to keep returning to your web site designers or developers for every little thing. With your content management system, you can just log in and update key information based on your business needs, saving time and money, and reducing (if not eliminating) delays.