In the xml file of Android Studio, what does 'android:layout_below' mean in the phrase 'android:layout_below'?

Think of it as a variable (prefix) that distinguishes the namespace defined by a specific uri.

xmlns:android="http://schemas.android.com/apk/res/android"

where android is the prefix followed by http://schemas.android.com/apk/res/android is uri. You define it at the top and use the android prefix like android:layout_below.

See also Manage Namespaces in XML documents~