A/R (Accounts Receivable) and A/P (Accounts Payable) are used by businesses to record sales for which they are not immediately paid, or to record bills that they have received, but might not pay until 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 most home users, A/R and A/P will add so much complexity that they are not worth the effort.