Android L has introduced something called a Floating Action Button (FAB), which is basically a circle overlayed on top your application. This FAB can be clicked to do an action. In the Google+ application this is used to persistently allow the user to create a new post.
It basically looks like in the screenshot above, where you can see a circle with a play icon at the bottom of the screen. Faiz Malkani backported this from Android L to work on JellyBean and KitKat, and I took the liberty to port it to Xamarin.Android.
You can find the repository on GitHub along with a sample application, which the screenshot above is from.