C M S
The three letter acronym C M S is used to describe a Content Management System. WordPress is a Content Management System. Generally speaking most contemporary websites will be based on a C M S.
The essence of it is that the site is managed by some form of engine that responds to inquiries from a browser as to the content to display. The information that the site displays is stored within a database.
Early websites were built on HTML (Hyper Text Markup Language), which essentially were based on a set of text files. This is replaced by a system which distinguishes between content (stored in the database) and the look and feel (Managed by the theme and the C S S files).
One of the clear advantages of a C M S is that no particular software is required in order to add edit or delete content, simply from a browser. In earlier days we needed to have software loaded on the local machine and then upload the changes. The second great advantage is that more than one person can update the website, (subject to passwords) and you do not need to be in a particular place to upload content.