The Auto Top-up feature on your SMS Account ensures your account does not run out of SMS credits.
When your credits drop down below a minimum level which you have set, the account is automatically topped with credits to the set level and charged to your credit card.
You can set this up here: Settings > Billing > Low credits & auto-topup. The minimum level you can set the auto-top up to is same as allowed on your plan.
However, a heads up that this feature does not work in case of a booked campaign that has insufficient credits. For booked campaigns, it is advisable you top-up credits manually before they go out.
You can also set-up Low Credit Alert emails to someone other than the user of the account (eg: Accounts, Admin) to ensure alerts go to the right person.