What are Transaction Security Policies and how do they work?

Transaction Security (part of Salesforce Shield) monitors real-time events in the org and takes automated actions when specified conditions are met. Components:

1. Event: What to monitor — API query events, data export, login events, entity changes (CUD on specific objects).
2. Condition: The filter criteria (e.g., "a user queries more than 2000 records in one transaction").
3. Actions: Block the action and/or notify the admin/user. Policies are defined using either a point-and-click interface or Apex Policy classes (for complex logic). Common use cases: blocking bulk exports of sensitive data, detecting logins from unusual locations, enforcing MFA for specific operations.