What does the final keyword actually do in java and for android development?

I believe Dai has already given you an apt answer to the question however I would like to add to it based on three different uses that final offers in Java thus android.

1) Defining a variable - you can declare a variable as final.

final int USER_ONE = 1;

When we declare a variable as final its value will never change through out the entire program. Also by convention in Java final variables names are written all in capital letters

2) Declaring a method as final - you can also declare a method as final. When a method is declared final it cannot be overridden

class A {
  final void hello(){
      //some code
   }

class B extends class A {
  void hello() {  // this line will generate an error saying hello cannot be overridden
   //some code
   }

3) Declaring a class as final - when you declare an entire class as final that class cannot be inherited by another class

final class A{}

class B extends class A {  // this will give a compile time error since A cannot be inherited since its final
//class memebers
}

Hope that explains. Ask again if you need more info.