When you arrange a direct debit, you are essentially giving someone else permission to take funds directly from your bank account.
A direct debit authority is your instruction to your bank to accept debits from your account by another person or business (known as the direct debit initiator).
This is different from automatic payments.
An automatic payment is an instruction to your bank to make a regular payment of a fixed amount from your account to someone else, either for a fixed period of time or indefinitely. A direct debit allows the initiator to submit the specific amount to be debited from your account -- so the amount can vary each time.
You can cancel a direct debit authority at any time through your bank and your right to do so is usually set out on the direct debit authority form.