Using SMS Credits is mostly related to using SMS Gate feature.
SMS credit allows you to send messages to mobile numbers. For now, it is reserved for sending verification codes with the SMS Lock. This way you will safely lock your video and allow access only to viewers which provide a valid mobile phone number.
You can always check your current credits here:
And you can always purchase some more by clicking on the BUY SMS button.
Once you choose your pack you can just confirm your credit card if you already saved it or add a new one.