What Is ACH?

ACH = Automated Clearing House. 
The process is similar to writing and depositing a check manually without the physical check. 

ACH is a nationwide mechanism that processes electronically originated batches of high-volume, low-value credit and debit transfers (vs. wire transfers, which are utilized when the individual dollar amounts are larger; wire transfer is typically done in 24 hours; not done in batches).

ACH payments are electronic payments made through the Automated Clearing House (ACH) Network.  Rather than sending each payment separately, ACH transactions are accumulated and sorted by destination for transmission during a predetermined time period. This provides significant economies of scale and faster processing than paper checks. 

ACH payments can be electronic from start-to-finish, or they can happen when a paper check is converted and processed electronically.

While all national banks are subject to the same maximum hold periods established by law, each bank may make deposits available sooner. A bank can make the deposit available immediately or delay availability up to the maximum.