Answer
Segregation of duties splits key responsibilities—authorization, custody of assets, recording, and reconciliation—among different people so no single individual can both perpetrate and conceal an error or fraud. It's a fundamental control activity; its absence (e.g., one person handling and recording cash) is a common significant deficiency.
Real-world example The person who approves payments is different from the one who sets up vendors and the one who reconciles the bank, preventing fraud.