Answer:
Debits decrease the balance of the Accounts Payable account. Why?
Accounts Payable is presented in the Liability section of the Balance Sheet. If you purchase a printer for $200 and enter the bill into the accounting program, the program will debit the expense account you choose (e.g. Office Equipment) for $200 and credit Accounts Payable $200.
Then, when you pay the bill, the accounting program will debit Accounts Payable $200 - thereby canceling out, you might say, the earlier credit. (And the accounting program will, of course, also credit the Checking Account by $200.)
So when we enter bills into the accounting program, Accounts Payable is credited. And when we pay the bill, Accounts Payable is debited ... its balance id decreased.