The Get Credits screen shows the current Credit balance as well as gives multiple options to buy more Credits from within the app.
Credit balance is only required when you want to make a call or send an SMS text to a real number. In other words, Credit is a pass to enable you to talk to the world outside TextApp, using TextApp!
You can tap on a specific Credit pack and purchase using your real currency within the app. On successful purchase, the Credits will be added to your Credit balance.