ACH (Automated Clearing House), sometimes called EFT (Electronic Funds Transfer)
or simply "direct deposit", is a way to transfer funds between bank accounts
at different financial institutions, usually limited to the US. Generally transfers occur within 1 business day, though due to the way reversals happen, many financial institutions have an additional 1 or 2 business day hold to ensure that the transfer was successful.
If you want more information on how the ACH network works, there's a great article at ZenPayroll detailing all of the high-level and low-level details of it.