When a Flow hits an error, usually on an element that talks to the Salesforce database, the interview fails by default, the running user sees a vague unhandled-fault message, and Salesforce emails an administrator. A fault path is a secondary route that runs only if its source element fails, so you can show a clearer message, recover, or notify someone instead of stopping with the generic error. Error emails are the main troubleshooting record of what ran, which element failed, and what data was in play.