Apache Cordova is a tool/library/… for writing cross platform (mobile) applications. With Cordova you write the code in HTML5, CSS and JS like you would on a homepage, just packed into an application that can run on many different platforms.
But Cordova is more than just that, it also has a number of plug-ins that makes it possible to access systems on the platforms that a normal web application can’t access. It is even possible to make your own plug-ins. The plug-ins are typically written in the native language for the platform, like Java for Android, C# for Windows Phone etc.
Unfortunately the “official” Cordova documentation for writing plug-ins are…hmm…aeee… let’s just say, I couldn’t figure it out, but thanks to Google I found this blog post that describes how to write plug-ins in Java for Android.