The Credit is the name (term) of the balance units used by the app to make calls or send Text messages (SMS). 

You can recharge your credit balance from within the app just with a tap of a button. 

Also, there is a way to earn free credits which you can use without paying a penny. The free credits can be earned in exchange of watching the advertisements offered by the app's sponsors.

In case you are looking for ways to recharge/refill your Credit balance, here is the relevant article.

You have a good alternate too! You can get additionally subscribe to the Gold Plan to make calls & Texts without worrying about Credit balance much.