This is an interesting video about Floating Action Button made by Webucator about Android Promoted Actions: Floating Action Button.
They are actions that are directly visible in the UI instead of using action bar button; for this reason these actions are called promoted, they can be easily accessed and define the main action in the current UI. For example, if we are using an email app and we are listing the inbox folder, one promoted action can be a new mail. The visual pattern is called Float Action Button (or FAB) because the promoted action can be represented using a floating circular button on the UI.
The picture below shows it:
This video is made using my post explaining how to use FAB.
Find more videos @ Webucator