SMS credit is an allocation of SMS (text messages) that you're able to send to a mobile phone:
Initially, you're granted a credit of fifty messages.
Every time you send a message, your credit decreases by one.
Every time you receive an SMS message in Chat (for example when a phone user replies to one of your messages) your credit increases by five, up to a maximum of 50.
If your SMS credit goes down to zero at any point, it will increase back up to one 24 hours later. So, you won't ever be locked out of the system.
"Buying" additional messages
Keep in mind that if you'd like a higher message credit, you can always send an SMS to your own phone, and then reply to that message multiple times. Every time you send a reply message, your SMS credit is increased by five. Effectively, you're buying more messages by paying your phone company for these outgoing messages.