This stage flags orders that have hit an issue during fulfilment or delivery - an inaccessible address, a customer not answering, damaged stock, or anything else preventing normal completion.
Go to Dashboard > Workflow > Order Workflow and look for the Problem Orders column.
Add a note explaining the issue, contact the customer if needed, then move the order back into the workflow once resolved (re-attempting delivery, rescheduling, or processing a refund or credit note if appropriate).
Keeping problem orders visible in their own column means they cannot get lost or forgotten among orders that are progressing normally.