What is an FDR, and what is it used for in the novel, Airframe?

The flight data recorder, or FDR, is a device that records the state of an aircrafts system during flight and can be valuable in learning what happened prior to an accident. However, FDRs often fail during flight and are notoriously unreliable. In the case of Flight 545, one of the sensors stopped working, and the team is unable to recover the information they need to determine the cause of the accident.