Prototyping is when you make the simplest version of your ideas in a quick manner to prove if your concept works well or not. For some people, they will do this via index cards, paper, chits, and boards, which is called a paper prototype. This can be quite useful, as you don't have to think about the code side of things to begin with, and instead it allows you to experience the core of the game without having all the nice art and polish. A game that is fun to play with bad graphics will only be improved when you add to it.

Of course, assuming you have purchased this book, you are probably already a developer, but it's still a good idea to think of it as an option. Jesse Schell wrote about paper prototypes in his book The Art of Game Design: A Book of Lenses, where he explained how you could create a paper prototype of the game Tetris. To do so, you could cut out cardboard pieces and then put them into a pile and pick them out randomly, and then slide them down the board, which would be a piece of paper. Once you completed a line, you would grab an X-acto knife and then cut the pieces. While it doesn't give you the same experience entirely, it captures enough of the experience to see if you are using the right kinds of shapes and how fast the pieces should drop. The big advantage would be that you could create this prototype in 10 to 15 minutes, while programming it may take a lot longer.

It's a lot easier to justify 30 minutes of work rather than a day for something that doesn't work out. This can be done with 3D games as well, such as First Person Shooters, by creating maps in a similar way to how you create combat encounters in a pen and paper role playing game such as Wizards of the Coast's Dungeons and Dragons (which is a great thing to learn how to play as a designer, as you learn about telling a story and developing interesting encounters).

A prototype's job is to prove if your game works and, specifically, how it works. Don't invest in only one idea specifically, but rather create a number of small prototypes that are quick to make, without worrying if it is polished or you've made it the best possible.

One of the most vital skills as a game developer is being able to create rapid prototypes, to see how it works, and then test it. We call this process of testing out a game idea playtesting.