Eventually, you will receive payment from your customers for outstanding invoices. To register these payments, use the Process Payment application found in Business -> Customer -> Process Payment.
The Process Payment application consists of:
Payment Information - Customer - the customer who paid you.
Payment Information - Invoice - the invoice for which payment was received. When you select an invoice, the Amount field is set to total due for the selected invoice.
Payment Information - Date - the date you you received payment.
Payment Information - Amount - the amount of money received.
Payment Information - Num - the check number.
Payment Information - Memo - any comments about this payment.
Post To - the A/Receivable account to which to post this transaction.
Transfer Account - the account where the money will be deposited (a checking account for example).
Below is the GnuCash Accounts Receivable payment window.
Process Payment Window