In Craft CMS you can name your template files whatever you want, about.twig, super-banana.twig, etc.
You can then enter that filename into the url to load that template directly:
This is useful if you need to create a page that doesn't have to be edited via the Craft CMS control panel, but rather pulls in content from external sources or other sections (ex: blog archive page, blog index, the Homepage).
If you however prefix the filename with an underscore (ex: _about.twig), the template cannot be loaded directly via the URL.
You can change the prefix that is used to indicate that a template is private, by changing the general config option privateTemplateTrigger to something like "private_" to make it super obvious.
And that's pretty much all there is to it.