A/R (Accounts Receivable) and A/P (Accounts Payable) are advanced concepts that are used by businesses to record sales for which they are not paid right away, or to record bills that they have received, but might not pay until a little while later.
These types of accounts are used primarily when you've got a lot of bills and receipts flowing in and out, and don't want to lose track of them just because you don't pay/get paid right away.
For almost all home users, A/R and A/P are too complicated and confusing to be worth the effort.