Debit and Credit are from accounting. Debit means to increase asset or decrease liability, while Credit is the converse, to decrease asset or increase liability.
When you have made a payment to bank/credit card, you decreased the bank's asset(the money you owed the bank), or when there is excess payment, the bank owes you money and hence their liability increased. Hence, payment to banks are considered credit and thus positive balance in your favour are credit balance. Note that the credit debit are usually taken with respect to the bank, and not customers.
If the above is too much for understanding, just take innumerabilis explanation.
CR is credit, which is what the bank owes you. If you have 100CR you don't need to pay the CC bill.