What you have there is a class definition. Class definitions are blueprints for objects.

When you create an object of a class you call its constructor and it reserves space in memory  for all the values used in the object.

So in your class definition below, you're saying that all objects of Begin will have an interger a, that will need to be reserved.

So to answer your question, there would be a line such as:
Begin test = new Begin();
test.setup();
test.draw();
...
This is probably in a main method definition somewhere else. Generally speaking you don't have to worry about Processing handles this. Its more important if you want to create your own class/object definitions. The processing crew have tried to simplify the interface as much as possible.