CMS
A software platform for creating, managing, and publishing digital content — adjacent professional vocabulary.
In practice
A content management system lets people add and edit the text, images and pages of a website without touching code, through forms and editors. It matters to designers because real content will be entered by editors for years, not by the designer: headlines may run longer than in the mockup, and images may arrive in odd shapes. Designing an article page means planning for the shortest and longest headline the CMS allows.
Often confused with Template
A CMS is the software that stores and publishes content; a template is a layout that the CMS fills with that content.
Try this
Rewrite the headline of a website mockup at three times its length and check whether the layout still holds.