Universal Apps in short is an application model that Microsoft invented for applications that is being served in Windows Store. This enabled the application to be served in both Windows Store and Windows Phone Store. It is not a really universal application cross all platform, but it is universal in Windows world only but this application model brought the ability for developers to target all windows 8/8.1 devises while using the same code over and over; this include windows phones, tablets and PCs..What enabled this is having the Windows Common Runttime; the universal apps is being built on top of this new runtime which is already there for different windows platforms.
if you are interesting to know more about what is the universal apps, I’m planning to write more articles about it but if you can’t wait, I advise you to read this article on this link.