An exit interview is a conversation with a departing employee to understand why they are leaving and gather honest feedback about their experience.
An exit interview is usually held in the final days of employment, in person or through a short survey. It covers why the person is leaving, what worked, what did not, and what would have made them stay.
People leaving a company often give the most candid feedback you will ever hear, because they have little to lose. Patterns across many exit interviews point directly at the things driving good people away, which makes this one of the most useful inputs for improving retention.