It defines an XML Namespace.
In your example, the Namespace Prefix is “android” and the Namespace URI is “http://schemas.android.com/apk/res/android“
In the document, you see elements like: <android:foo />
Think of the namespace prefix as a variable with a short name alias for the full namespace URI. It is the equivalent of writing <http://schemas.android.com/apk/res/android:foo /> with regards to what it “means” when an XML parser reads the document.
NOTE: You cannot actually use the full namespace URI in place of the namespace prefix in an XML instance document.
