An exit interview is a conversation with a departing employee about the reasons for leaving and their experience of working at the company. It is the cheapest source of information about what is not working, because someone who is already leaving has no reason to sugarcoat things. The condition is that the interview must not be conducted by the direct manager, who is often part of the problem, and it must be clear that the answers will not affect references. Ask specific questions: what was the main reason for the decision, when did it first appear, what would need to be different, what does the company do well, and what should the successor know. Real value only emerges once the answers are collected systematically and recurring themes are evaluated once per period; a single interview on its own changes nothing.
See also: Employee turnover and exit costs, Employee onboarding, Employer Branding.