CMS
A CMS is a screen where you change the words and pictures of your site without touching any code.
In a simple example
You open a page, fix the price of a cake, press Save, and the site shows the new price.
Why it matters to you
You can keep your own site up to date without paying someone for every small change.
Do not confuse with
A CMS is not the site itself. It is the control screen behind it.