CMS stands for “content management system”. It is a program that lets you build a website and keep it up to date through a simple screen in your browser, without writing computer code by hand.
Think of the difference between building a car from separate parts and simply driving one: instead of assembling a site piece by piece, you log in, type your text, drop in a photo, click save, and the CMS turns it into a finished web page. That is why a business owner can run their own site without being a programmer.
The most popular content management system in the world is WordPress. A CMS still needs a place to live — Hosting — and a name people type in the browser — a Domain.