A content management system is a piece of software which runs a web site, allowing the site’s owners to edit the site without technical knowledge.
CMS packages are extremely varied in price and capabilities, and most offer many more features above simple editing. Your choice of CMS, if any, can greatly effect what can be done with your site in terms of SEO. The most popular CMS on the web is currently WordPress. Other common examples include Joomla! and Drupal.