Layout
A layout defines the structure of a user interface in your application.
All elements in the layout are built using a hierarchy of Views and ViewGroup objects. A View usually draws something the user can see and interact with. Whereas a ViewGroup is an invisible container that defines the layout structure for Views and other ViewGroup objects, as shown in figure.(We will see more about ViewGroup)
So, to make it very simple we can say that android uses certain types of technique called XML(Extensible Markup Language) to define all these types of Views and ViewGroup Objects. Android provides a straightforward XML vocabulary that corresponds to the View classes and subclasses, such as those for widgets and layouts.
You can also use Android Studio's Layout Editor to build your XML layout using a drag-and-drop feature.