CMS stands for Content Management System. A CMS is a platform used to build websites that are easily edited by multiple users.
They are usually based on databases which hold the text content of the site. They also use templates so that pages share a common layout. The site navigation is derived from the database too, so that if a new page is added in a given section then links to it appear on all the places where it should.
Some CMSs will have support for workflow, so that users may not be permitted to publish pages, but only create them for other users to approve and publish.