Pretty much in in the title.
Have seen this around and after a bit of searching can't seem to find out what it actually does or what indeterminate progress is.
requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS)
It's used for example by RoboSpice samples.
Update: Currently this feature is deprecated and not supported on current devices, check the official documentation.
Original answer:
Simply rotating circle in top right corner in Actionbar which represent some background action.
Screenshot is from Sherlock Actionbar demos.