The difference between current balance and available credit is that the former refers to the amount you owe on your account, while the latter is the remaining amount of your credit card's credit line that you haven't spent yet.
Here's more info about current balance and available credit:
Current balance
The current balance reflects how much you actually owe in total, minus any pending purchases or payments. All of the purchases you've made that have been processed by your credit card company since you last paid your bill are included in the current balance.
Available credit
The available credit is the amount of your credit limit you can still use for purchases - how much you have left to spend, in other words. Once the account balance reaches the credit limit, the account has been “maxed out” and the available credit is zero. You can find your available credit on your monthly account statement or by logging in to your online credit card account.