This the Customer's TOTAL RECEIVABLE balance and is calculated by adding all debits from open invoices, minus any credit/refund invoices and unallocated/unreconciled payments.

You can see it on the Accounting tab of the Customer record, and (when Invoices are open) via the Customer Invoices screen or the Aged Partner Balance report.

You import these balances by importing invoices, or at least a Journal Entry that debits an Accounts Receivable account, and credits the Sales or Revenue account for the same amount.