When applied to websites, CMS refers to site in which the content can be edited by the customer themselves. The alternative is for all changes to be made by the web designer and charged pro rata to customer.
With a CMS site words, images, links, news items, products can all be changed easily. Whole pages can be added or removed, even turned off temporarily. Navigation menus are updated automatically to reflect changes made - essentially editing a website becomes no more difficult than editing a word processor document.