This attribute is used to configure the template that is used to render the page title when the controller tagged with this attribute is accessed:

// ...
use Oro\Bundle\NavigationBundle\Attribute\TitleTemplate;

#[TitleTemplate("The page title")]
public function demoAction()
    // ...

You can use arbitrary placeholders here which you then have to replace later on in your template using the oro_title_set() Twig function:

{% oro_title_set({params : {"%username%": fullname }}) %}