A simple explanation is that an item is cleared when a second activity occurs that zeros it out. Using A/R as an example, an invoice to a customer is cleared when the customer’s payment is posted against the invoice (txn F-32 -> Accounting > Accounts Receivable > Account > Clear). It can also be cleared with a credit memo. Similarly in A/P, a vendor invoice is cleared by posting a payment against the invoice (txn F-44). You may want to consider SAP’s AC-200 course which discusses accounting processes, including clearing of A/P’s and A/R’s.