Lock with and arrow around it at the top of the screen

blondie71197, a padlock with a circular arrow around it in the status bar at the top the screen means that the phone is locked to portrait mode and will not rotate to landscape mode. This can be turned on and off by double clicking home so that your open apps are displayed then swiping these icons to the right until your iPod controls appear, you should see an icon with a circular arrow. This icon locks your screen rotation. Good Luck and I hope this helps

I was asked to update this answer since with the advent of iOS 7 the method has been modified by something very similar. You now have to swipe up to bring up the control center, and then click the lock/unlock icon on the top-right of the menu to engage/disengage the screen lock.