While a game is being played, you Monitor whether it’s doing what it’s supposed to do, whether it’s going according to your plan, the vision you had in your head.
Control is when you monitor something that isn’t going to plan, you do something to fix it, to make it work the way you want to.
Successful changes go into the Replan, where you modify your prototype. Then you go back to Execute and you play it again, and you keep going round and round on that, gradually making your game better.
I despise the word “iterate”. Yes, this is an iterative (repetitive) process, but the word iterate, which is often used in video games, must be one of the ugliest words in the world, yet only covers half of what you’re doing. You are modifying and testing, not just playing again and again. The scientific method is involved. To be termed scientific, a method of inquiry must be based on gathering observable, empirical and measurable evidence subject to specific principles of reasoning. A scientific method consists of the collection of data through observation and experimentation and the formulation and testing of hypotheses. (Wikipedia)
Game design is lot more than that, though. Unlike scientists, in most cases you have to rely on relatively few tests. (Nowadays in video games we see “open beta” testing, and testing after release, in order to increase the sample size and use statistical methods of analysis.) Unlike the scientist you’re making changes in the design, an actual product, as well as experimenting to see what happens. Fortunately, this is usability testing, not scientific testing, and usability testing does not require a large number of trials. I strongly recommend that you check out the Nielsen Norman Group’s website at alertbox.com, and read their articles. They are talking about web design usability, but most of what they say applies to game design, especially video game design where user interface is very important. We have user interfaces in tabletop games, but they have over many centuries settled down and don’t change rapidly.