The Short Version
A CMS, or content management system, enables a website to be easily updated by someone who doesn't have the training or background of a Web developer.
The Detailed Version
In an effort to give more control over to their clients, some web developers will create websites that are integrated with a CMS. A website that has CMS built-in can be edited (to a certain degree) by the owner or administrator of that site's company without having to contact a web developer for help.
While there are several CMS products available, many feature the same basic types of functions. After logging-in, a nontechnical user can add text, images, links, and in some cases, even meta content (such as page titles and descriptions). In addition, the CMS will usually permit edits to the text's font and style.
In most applications, a CMS cannot be used to change core elements of a webpage such as background color, page layout, navigation menu, or header/footer placement. More significant changes like these are still typically made by full-time experts, and require extra time and work.