Laravel blade strange execution sequence [closed]

I came across a Laravel tutorial. It’s about authentication system and creating login interface.
There is some strange code that I don’t understand.

There is a login form submitting a post request to server to authenticate. Based on input data, it may fail with errors (Like invalid email or …). There is a field under the form to show those errors:

@if ($errors->any())
  <ul>
    @foreach ($errors->all() as $error)
      <li>{{ $error }}</li>
    @endforeach
  </ul>
@endif

PHP directives execute on server side (?), so how it shows errors after submitting the form? Page is already loaded. So how it works?