Not an expert - hopefully Genghis the Engineer might chip in at some point - but Reliability Engineers usually work within the Technical Services or Quality Assurance Dept. of a JAR 145 maintenance organisation and collect and analyse a lot of data both from the Aircraft Flight Data Recorders and from other sources such as condition reports on worn or failed components in order to monitor the continuing reliability of aircraft systems and components. If trend analysis shows that things are wearing or failing sooner than expected, or perhaps are actually lasting longer than expected the maintenance schedule for an aircraft can be amended to optimise the maintenance intervals. This can both help to prevent critical failure modes developing within a system unexpectedly, which could lead to a serious accident, and can save the airline money if unnecessary maintenance can be minimised.