An order approval is created when Send Purchase Order parks an order instead of sending it, because the order would take the day's spend with that supplier for that outlet over a configured daily threshold. The order waits in AWAITING_APPROVAL until a user with the Approve Orders permission approves it, which sends the order to the supplier, or rejects it.
Thresholds are set on the library, on a supplier, or per outlet in a supplier's ordering details, and only apply to libraries with the Order Approval module.
In the Apicbase app, they are listed on the Order Approvals page, where each one is called an approval request.