Zakaria, that is a reference to an built-in XML layout document that is part of the Android OS, rather than one of your own XML layouts.
Here is a further list of layouts that you can use:
http://developer.android.com/reference/android/R.layout.html
(Updated link thanks @Estel: https://github.com/android/platform_frameworks_base/tree/master/core/res/res/layout )
You can actually view the code for the layouts.