Direct Debits are a payment method that allows merchants and any other third party (biller or beneficiary) to get paid by directly pulling funds from your account on an agreed date and with your prior authorization. The amount billed may vary from period to period (i.e. telephone bills).
They are normally used by companies that offer recurrent services with recurring payments (some examples are credit cards, utilities, cable providers) since it makes them more convenient, minimizes errors, and helps avoid late payments. They are also used by companies that want to offer their customers an alternative payment method to credit/debit cards, and want to complete the payment process within their apps / web (i.e. digital wallets, etc.)