Auto-recharge is a setting that will automatically add funds to your account when it falls below a certain threshold.
This feature is available to ensure that your messages will not be interrupted. Once your balance falls to $0, then any incoming and outgoing texts or calls will stop.
In this example, when your balance falls below $10, it will recharge the balance to $50 using the credit card selected.
You can also setup a Backup Payment Method so if this card fails, it will attempt the backup card to ensure that funds are added.