An application has to meet various requirements in order to be useful. There are functional requirements (what it should do, such as allowing data to be stored, retrieved, searched, and processed in various ways), and some nonfunctional requirements (general properties like security, reliability, compliance, scalability, compatibility, and maintainability)
Reliability means making systems work correctly, even when faults occur. Faults can be in hardware (typically random and uncorrelated), software (bugs are typically and systematic and hard to deal with it), and humans (who inevitably make mistakes from time to time). Fault-tolerance techniques can hide certain type of faults from the end user.