ACH payments are bank-to-bank transfers. ACH stands for the Automated Clearing House, which is the electronic network used for financial transactions in the United States. This is essentially the "highway" that money is electronically routed through to get from bank to bank.
ACH is a way to move money between banks without using checks. ACH payments are processed only on business days. If a renter schedules a payment on a weekend or bank holiday, the payment will begin sending on the next business day.