It means that you've got rotation lock 'on', so the screen doesn't turn if you rotate the iPad. Depending on what you've got Settings > General > Use Side Switch To set to (rotation lock or mute notifications), then you can lock/unlock the rotation by the switch on the right hand side of the iPad above the volume switch, or via the taskbar : double-click the home button; slide from the left; and it's the icon far left; press home again to exit the taskbar. The function that isn't on the side switch is set via the taskbar instead : http://support.apple.com/kb/HT4085
Just in case you haven;t, I would recommend a read of the iPad user guide.
It explains this and many other things.
It can also be obtained from within iBooks for free.