ACH stands for Automated Clearing House, the network U.S. banks use to move payments between accounts. An ACH debit pulls money out of your account on a date and in an amount you authorized, usually in the funding contract.
Fixed-payment advances usually collect this way, every business day or once a week. The debit runs whether or not sales came in that day, so the balance has to be there. A debit that bounces is returned, and both your bank and the funder may charge a fee.
Blocking or stopping the debit without the funder's agreement is usually listed as an event of default. If sales drop, ask the funder to reconcile or lower the debit first, and keep the request in writing.