To open a direct debit, the organisation needs your signature. Without a signature, there is no valid mandate. Because you give your agreement in advance, your expenses can be paid automatically without you having to intervene. With a direct debit, the initiative for the payment no longer lies with you, but with the organisation that requests money from your account.
You can use a direct debit both for repeated payments (= a repeated direct debit; e.g. monthly or quarterly) and for a single payment (= a single collection). Direct debits can be used both nationally and internationally - between countries in the SEPA zone. In case of a European direct debit, it is the suppliers who manage your direct debit mandates.