A content management system is what I’m typing this on right now. It’s a system that allows someone to create a website without, in theory, writing any code. The CMS I have personal experience with are WordPress, Joomala, Microsoft SharePoint, Google Sites, Moodle LMS, and Squarespace.
Content management systems changed web design by separating web design and web development. They allowed amateurs to create at least semi-decent websites, and they allowed professional web designers to focus on designing, with coding becoming a secondary skill. It’s still important to know how to code in HTML, so that you aren’t stuck when the CMS doesn’t want to play nice and something needs to look just so.
At the same time, the web developers became actual programmers who made websites do more and more things that they hadn’t been able to do before, giving us web designers more toys to play with.
Creating a site in Dreamweaver means building it more or less from the ground up, in HTML and CSS. It’s rewarding to know that you can make a site from scratch, but it’s also usually not the best way to do things in 2020. For most sites a CMS will let you build a more attractive site faster.