Instead of defining all of your html inside of your controller you may wish to organize it into template files. These files, referred to as a views, are stored in the resources/views directory. Views are powered by the Twig templating language. You can read more about Twig at: http://twig.sensiolabs.org/
Here is an example of a basic view:
<!DOCTYPE html>
<html>
<head>
<title>My Webpage</title>
</head>
<body>
<h1>{{ title }}</h1>
<p>{{ content }}</p>
</body>
</html>
Now, if we save the view at resources/views/demo/example.twig, we can call it from our controller like so:
return view('@MyPlugin/demo/example.twig', [
'title' => 'My Demo',
'content' => 'Congrats on your demo view.'
]);