About Singleton design pattern: http://en.wikipedia.org/wiki/Singleton_pattern
About context i'll give you for example game engine:
Game engine is created as a singleton. To get the engine object i run GEngine.getInstance();
The engine contains GameView and Android Activity Context objects. So from anywhere I can get GameView or activity Context with a code: GEngine.getInstance().getGameView() or GEngine.getInstance().getContext().
This is just a simple example, I don't want to expand myself to much here. It's just a standard practice used all around.