How does server decided that the user asked a protected resource?

The following quote is from Spring security documentation:

Consider a typical web application's authentication process:

You visit the home page, and click on a link.

A request goes to the server, and the server decides that you've asked for a protected resource.

[...]

It's not clear to me how the server decides that the user asked for a protected resource? Is there a special filter for that o something similar? If so, couldn't you point me out to the class?