What does onPrepareOptionsMenu do?

onCreateOptionsMenu is called once, when your activity is first created. If it returns false, no option menu is shown and onPrepareOptionsMenu is never called.

If onCreateOptionsMenu returns true, onPrepareOptionsMenu is also called before the activity is displayed, and also every time the options menu is invalidated. Use onPrepareOptionsMenu if you need to enable/disable, show/hide, or add/remove items after creating it.

If your menu does not change, use onCreateOptionsMenu.

If you want to alter the menu before it's shown to the user, you can put code to do that into onPrepareOptionsMenu. I've used that dynamically to disable some menu options in some circumstances.

As an example of when one might want to disable a menu option, I had an app where there was a way of specifying a destination. One of my menu options was to calculate a route to the destination. However, if a destination wasn't specified, that option didn't apply, so I used onPrepareOptionsMenu to disable that menu option when it wasn't applicable.

From Android 3.0 and beyond, there's the ActionBar, which is a menu bar. The most important items go into the ActionBar itself, but then there's an overflow for when there's not enough room on the action bar. One can specify that menu items should always be in the overflow menu and never on the action bar itself. On some devices, the action bar overflow corresponds to the permanent menu button on the device, whereas on other devices which don't have a menu button the overflow menu is seen on the right hand side of the action bar as three vertical dots.

Take a look in the API:

Prepare the Screen's standard options menu to be displayed. This is called right before the menu is shown, every time it is shown. You can use this method to efficiently enable/disable items or otherwise dynamically modify the contents.