In Android the visual design is stored in XML files and each Activity is associated to a design.
setContentView(R.layout.main)
R means Resource
layout means design
main is the xml you have created under res->layout->main.xml
Whenever you want to change the current look of an Activity or when you move from one Activity to another, the new Activity must have a design to show. We call setContentView in onCreate with the desired design as argument.