Depending on your Settings, it's either a Mute button or a Rotation lock button.
You need to check how you've got it set in your Settings app. Then, remember, whichever setting you have it on, a double-tap of the Home button (or a four-gesture swipe up) will reveal the multi-tasking bar. Then, swipe you hand right so that you bring the left side of the row to view. The furthest icon will control the function the button does not. In other words:
* If you've set the button to be Mute, the swipe feature will reveal an icon to lock the iPad's orientation (it won't rotate)
* If you've set the button to be the orientation/rotation lock, the swipe feature will reveal an icon to mute the iPad sounds
Hope this clarifies.
Marilyn