The Chats screen is a central place to stay in a conversation using your Private Phone Number as well as TextApp number.
Both these types work through a wi-fi or mobile Internet connection.
Chat - Have conversation with other TextApp users (whose number starts with '+T 123') without any extra cost.
SMS - Have conversation with people using their real world phone numbers (having an ISD Code) by using your Credit balance. The recipient need not be a TextApp user.
Chats > Chat
Start a new conversation, or respond in an existing message thread to people using their TextApp number (that starts with '+T 123'). You can send image or text messages and they will be delivered to the TextApp at the receiver's end. You will not need to pay anything additional to send or receive Chat messages.
Chats > SMS
Start a new text conversation, or respond in an existing message thread for your Private Phone Number, just like how you communicate with a phone number (even if the recipient does not have TextApp). Your messages will be delivered instantly to the recipient and s/he can read them just like a regular SMS (text) from anyone else. Please be aware that your Credit balance will be used when you send an SMS.
In case you are looking for options to recharge your Credits, here is the article.