Simply put, a content management system allows you to manage and control the content on your website. If you can use Microsoft Word you will likely find adding, changing and managing your website content a simple process.
For a more detailed description from Wikipedia:
A content management system (CMS) is the collection of procedures used to manage work flow in a collaborative environment. These procedures can be manual or computer-based. The procedures are designed to:
In a CMS, data can be defined as nearly anything - documents, movies, pictures, phone numbers, scientific data, etc. CMSs are frequently used for storing, controlling, revising, semantically enriching, and publishing documentation.