A direct credit is an electronic deposit to your account. For example, your employer may deposit your salary into your account or you may allow Telstra to deposit your share dividends into your account.
A direct debit is an electronic withdrawal from your account. For example, you could give your insurance company authorisation to withdraw your regular premium from your account when it is due.
You do need to ensure funds are in your account when direct debits are due. Otherwise, a direct debit dishonour fee will apply if the payment is declined due to insufficient funds.