At the games development if you paint animations at the screen or you are scrolling the scene, you want it to look smooth. To do that you need paint certain amount of pictures at the screen per second. This is called Frames per Second (FPS).
When you have a PC/Mac game you usually have to buy better hardware or decrease detail quality to raise the FPS to level that it will be nice.
But if you have the iPhone it’s up to the developer to create the game in such quality to look and works nicely. (if you have Android you have more choices to buy so you can still upgrade).
But what is the optimal FPS? Well let’s put little of the technical details.