What is Debit Memo and Credit Memo in AR?

In AR Debit memo is Positive Amount for example we are selling the product to the customer.
Either we may forget to add a freight charges or some other thing.  So at that time we are prepare or Rise the Debit memo it is increased the Org balance.
(Customer is Under Charged at that time Org prepare Debit memo)

In AR Credit memo is Negative Amount if you billed more than your customer then Org need to raise Credit memo to give the credit to your Customer, so it is decreasing the Org balance.
(Customer is Over Charged at that time Org prepare Credit memo)