allowfullscreen is an HTML attribute that enables videos to be displayed in fullscreen mode.
The following HTML code, when inserted into a webpage, displays a YouTube video with a fullscreen button:
<iframe width="560" height="315" src="http://www.youtube.com/embed/9bZkp7q19f0" allowfullscreen></iframe>
Here is what the code looks like when displayed to a user. Once the video is played, a fullscreen button is displayed in the bottom-right hand corner: