The Settings screen shows your WiFi & Sim phone numbers and also enables you to customize themes, refer to Call & SMS rates, etc.
WiFi Number - is the number that starts with '+T 123' is your WiFi number. Any TextApp user can call or message you on your WiFi number for free. Similarly, you can also contact any WiFi number.
Sim (Private) Number - is the number that is as good as any phone number that exists in the real world. Anyone can call or SMS you on your Private Number and you will receive them via TextApp. Similarly, you can contact people on their phone number using your Private Number. When you make a call or send a text using your Private Number, Credits are deducted from your Credit balance, depending on the length and destination country.