A CMS (content management system) is geek speak for an online user interface that allows you to log-in and edit site from anywhere in the world with a web browser and internet connection. You're not tied down to any particular software. The content of your site is stored in SQL databases and pulled into pages on the fly by server-side programming like PHP. In all likelihood, this is not something you could build yourself. It requires skill with advanced code and web server security. But it makes the site much easier for non-coders to maintain.
To give you an example, WordPress is a popular open source CMS. It's free and a lot of people use it to run their blogs and web sites.
Nancy O.