Content Management System is a software to manage Contents and related things on a web site or system dynamically.
Before web programming languages were not advanced as it today, web sites were used to be built static with HTML. During the time it's evaluated with people are started to develop software on their requirements and became a part of the ecosystem with a label "CMS".
I think "WordPress" was the biggest thing to make "CMS" popular all around the world and made people understand how it's easy by developing these kind of interfaces.
Nowadays with advanced programming languages it's very easy to develop even your own CMS rather than using ready structures.