We might divide a page template into a header, content, sidebar and footer where these are in separate files. The content will change most between pages, so rather than having a content file, we include the content into a page file which pulls in the individual template files and defines the unique features of that page. The unique features of a page will be the title, description and article content. We can define these at the start of the page template with say PHP code to set the value of variables that are used by the other templates such as the header template. Then we assemble the page by including the required templates and inserting the content.
CMS helps to separate design of web site from its content. It means designer develops a web site and gives it to its owner or manager who can fill a certain design with different content. If necessary the content of web site, its images and graphics can be easily edited without intervention into design of web site. All changes become visible as soon as you make them. (Don‘t forget to reload page after making changes). Even if you are not an expert of HTML coding, you can easily manage the content of your web site yourself.