This is a memorandum record. That is, the Receivables Ledger does not form part of the double-entry system. It is a way to keep track of which customers owe you money.
An account is kept for each customer. Sales invoices issued to the customer are Credited to their account (it is not a double-entry - it just mirrors the Cr entry to the main general ledger Receivables Control account).
Similarly, receipts from the customer are Debited to their individual account.
The difference between the two sides is the amount you are still owed by that customer at that point in time.
Later on, you will see how the reconciliation between the individual Receivable accounts in the Receivables Ledger & the main Receivable Control account in the General Ledger provide a check for errors.