The following code snippet adds an AdView to a layout. What does the XML attribute ads:adUnitId refer to?

android:layout_width="wrap_content"

android:layout_height="wrap_content"

ads:adUnitId="MY_AD_UNIT_ID"

ads:adSize="BANNER"

ads:testDevices="TEST_EMULATOR, TEST_DEVICE_ID"

ads:loadAdOnCreate="true"/>