(MGDA means "Making a Game: the Direct Approach" and it is a series of blog posts meant to educate people to learn how to make a video game professionally.)
To advance to this phase, you should now have a potential idea that you would want to develop into a video game. Preferably something you want to do with a passion.
In this phase, your task is to turn that idea into a functional game concept. You do this by making all of its features as clear as possible for yourself. This is so you know exactly how the game looks and functions in its finished state and how are you going to get there. Thats the entire goal of the design phase.
During this phase, these words are most sacred: Focus on what matters.