Many modern applications are based on the SPA or Single Page Application model. From the users perspective, this means that the whole website looks similar to an application in a single page.
This is good because, if done correctly, it enhances the user experience, mainly reducing waiting times, because there are no new pages to load--the whole website is on a single page. This is how Facebook, Medium, Google, and many other websites work.
URLs don't point to HTML pages anymore, but to particular states of your application (that most often look like different pages). In practice, on a server, assuming that your application is inside the index.html page, this is implemented by redirecting the user that is requesting ,say, about ...
