An audit trail or audit log is a history of every transaction and includes all of the documents involved in the purchasing and payment process, including messages between employees, invoices, purchase orders, and more. It is a record of all the steps involved in the transaction.
You can think of an audit trail as an insurance. You pay it no heed or need it unless something goes wrong. It’s the same for an audit trail. You don’t need it until you really need it. But if you don’t have one, the consequences can be catastrophic.