有人可以用简单的术语解释“超文本作为应用程序状态的引擎”

This seems to be the REST principal that I've had the hardest time wrapping my head around. I understand that when desiging a rest api most of the effort should go into designing/describing hypertext for the application. Any pointers to real world applications of this principal ? How does atom protocol apply this principal ? Can some one explain that in simple terms how one would apply that to a hypothetical shopping cart rest api. This seems to be the REST principal that I've h