So, you're making a platformer. What's the first thing you do? Do you start designing the character sprites? Do you write the cut scenes? Do you start designing the level layouts and the challenges awaiting your hero-- the bottomless pits, the enemies, the disappearing platforms, all that fun stuff? In a word: no. For how can you create sprites if you don't know what they're supposed to represent? How can you write dialogue or a story if you don't know what sort of game play it's giving context to? How can you create challenges for the player if you don't yet know what the player can do?
Just as you can't start writing a college paper unless you have some idea what your thesis is, you can't start creating a game until you know its thesis-- its central big idea-- the thing that makes it stand out. In film making circles, this is known as the "high concept". High concept films can be summed up in a few words, and it is this pitch that is used to get
films greenlighted and into theaters near you.