GR/IR (Goods Received/Invoice Received) is an interim account. In the legacy system, if the goods are received and the invoice is not received, SAP makes a provision at the receipt of the goods. It passes the Accounting entry by debiting the inventory and crediting the GR/IR account.
Similarly, when an invoice is received, the vendor account is credited and GR/IR account is debited. Also, the GR/IR will show as an open item till the time the invoice is not received.