Answer
A reconciliation compares two independent records of the same thing to confirm they agree, and investigates/corrects any differences. In AP/AR it ensures sub-ledgers tie to the general ledger, ledgers tie to supplier/customer statements, and recorded cash ties to the bank. It's a core control that catches errors, omissions, and fraud.
Real-world example Each month-end the team reconciles the AR sub-ledger to the GL control account and resolves any differences before close.