CMS stands for Content Management System. Some people call these Web Platforms. You’ve probably heard of some of them: WordPress, SquareSpace, Wix. etc. The most popular by far is WordPress, which runs like 30% of the Web.
CMSs are applications that run on a web server that manage the back ends of your websites. If you’ve ever used a blog, you’re familiar with the basics. (At heart, all CMSs are blogs.) That is to say, they’re databases that keep track of all your pages, posts, and images that also provide relatively easy-to-use interfaces for creating the look, feel and content of your site. Modern CMSs extend their power by giving you tools to create the things you often see on the Web: slideshows, galleries, etc.
Sites hosted on a CMS work differently than old websites. In early days, developers would write an HTML file, and every time they needed to change something, they’d have to edit the code. That was called a “flat” site, and it was tedious and terrible. CMSs make editing much, much, easier.
When your site is built using a CMS, your page doesn’t actually exist until the user tries to access it, and which point the CMS will generate the HTML and send it directly to the user’s browser. (That’s how you can get customized content.) Almost every page you visit on the Web is generated on the fly.