A Content Management System is a web system for storing and displaying content.
Information we work with is usually stored in a database. A blog would have content tables that store paragraphs of text or formatted html. A basic CMS would support editing blog posts, setting titles and urls, and a page layout template.
Content Manangement Systems have been developed to solve common organizational problems.
Back when the web was composed of HTML files, someone would have to edit these by hand to add pages or update content. This prevented members of the organization from freely changing content themselves. CMSs were created to allow easy access for different members, with gated permissions and roles.