A content management system, often abbreviated as CMS, is software that helps users create, manage, and modify content on a website without the need for specialized technical knowledge.
In simpler language, a content management system is a tool that helps you build a website without needing to write all the code from scratch (or even know how to code at all).
Instead of building your own system for creating web pages, storing images, and other functions, the content management system handles all that basic infrastructure stuff for you so that you can focus on more forward-facing parts of your website.
CMS use php-scripts to generate the content to be sent to the client.
CMS is slower because of the hughes overhead. This is processed by the webserver.
Wordpress is primairy used for weblogs and news sites. For websites that use static content like this website, you beter code it yourself.
Nextcloud is a cloudserver that you can manage yourself.
Roundcube is a webmail server. It connects via imap to the mailserver.