Hey David,
CMS stands for content management system, which acts as a site builder i.e you don't need to code or use any web development knowledge to make your website. In simple words CMS is a software that provides you with an easy interface to make a website. It's is a simple process as making a power point presentation.
I found a similar post to cancel your confusion: https://www.branblog.tk/2018/05/things-you-need-to-know-about-cms.html
CMS require hosting installation, domain purchasing and more complicated management (databases, a variety of settings, safety support, manual updates etc.). Website builders are out-of-the-box tools, which don’t take any hosting setting effort, installation etc. You get all the features at a time.
You can think of CMS and website builders as readymade solutions to launch your website. However, if you need any customized functionality in your website then a CMS might not be the right choice for you. Try software development outsourcing to get a customized site built with ease.