Answer
A self-review threat arises when the auditor evaluates results of a service they (or their firm) previously performed, so they may not objectively assess their own work. Example: a firm prepares the client's financial statements or does its bookkeeping, then audits those same statements—reducing objectivity. Standards restrict providing such services to audit clients, especially public interest entities.
Real-world example A firm that designed the client's valuation model faces a self-review threat when auditing the resulting figure.