If your background is a Flash video / image / animation then the the answer is that not all mobiles support Flash and you need a Flash plugin for Android, for iPhones and iPads the situation is even worse - you need a different browser installing.
Designing and creating websites to display correctly on mobile devices can be a nightmare - most sites have mobile specific simpler styling and avoid things like Flash.
This still leaves the issue that many people will still not be able to view the site because they do not have a browser or mobile which supports Flash.
There are many scripts available on the Internet for Flash detection - you need to incorporate one of these into the home page that either pops up a message or reroutes them to another basic webpage which gives them information about using a browser which supports Flash - eg the one you have found - Puffin.