When your that code executes, Angular first lookup the HTML template in $templateCache with the id /Modules/Signin/signin.html.
If it doesn't find that in the $templateCache then yes, Angular will do an AJAX call using $http to get the HTML content and it will be loaded as normal resource which should be located at the URL like:
http://example.com/Modules/Signin/signin.html
You can verify it in your browser's developer's console that an AJAX call was performed.
Read more about $templateCache.
Basically, every template get's stored in the $templateCache when it is not stored already in the cache. So if you define the following in your index.html or any place (like where your angular is installed):
<script type="text/ng-template" id="/Modules/Signin/signin.html">
  <p>This is the content of the template</p>
</script>
Now as your Angular bootstraps, there will be a data in your $templateCache with id /Modules/Signin/signin.html so now your state code will not make any AJAX instead it will simply load the content defined above.