Auto-Refill ensures that your account maintains a minimum balance so that you can fund your pending investments faster without needing to wait for a wire or ACH to arrive after committing to an investment.
If you'd like enable Auto-Refills, click "Turn on Auto-Refill" on your Funding page next to your Account Balance. Once you connect your bank account under "ACH Bank Accounts," you can set a minimum balance.
If your AngelList account balance falls below that minimum balance, an ACH will be triggered from your linked ACH Bank Account. If you set a $5k minimum balance to maintain, the auto-refill ACH will also be for $5k.