The action bar is a persistent bar at the top of your application. You use this control to give users the abiity to perform certain actions from anywhere in your application.
You have the ability to swap out different icons, change the title, and more!
When you set your <uses-sdk/> tag to higher than Honeycomb (API level 11), you get this control for free. If you don't want the action bar, you can use the @android:style/Theme.NoTitleBar for the <theme/> tag in AndroidManifest.xml to hide it.