Who will buy and use your app?
Who are you aiming for? Are they younger? Older? Are they disabled? Athletic? Fans of a particular music genre? Social networkers? Businesspeople? Homeowners? etc.
If the audience is a particular segment of people, for example Star Trek fans or accountants, then what do you know about them? And what can you find out? What other apps do they probably have? Which websites do they visit? Is there an industry body or fan club?
How big do you think your intended audience will be? Can you find out the actual number or make a reasoned guess as to how many they are? For example Star Trek fans will register with fan websites, accountants have to belong to a professional body.
The estimated size of your intended audience is called your Market Size.
In as much detail as possible, list the types of people you think will use your app, and what you know about them.
Calculate your Market Size.