Most payments are a push: you tell your bank to send money out. A direct debit is the opposite, a pull. You sign a mandate once, and from then on the company (the originator, or "biller") instructs *its* bank to reach across the system and debit *your* account on a schedule. The three big batch rails that do this are ACH in the US, Bacs in the UK, and SEPA Direct Debit across the eurozone. They differ in detail but share one design: they are slow, cheap, batched, and built around a stored authorisation.