Mean Time Between Failure (MTBF) numbers represent predicted elapsed time between failures of a system during operation. The number is a statistical approximation and not intended to specify when a system will fail.
For example, if a system has an MTBF of 20 years, it does not fully means that device lasts an average of 20 years and then the system will fails. Statistically said, an MTBF of 20 years could be means that out of a total of 20 devices, one device fails within a one year period.
MTBF is used for repairable system (i.e. control system) while Mean Time to Failure (MTTF) is used for replaceable system (i.e. resistor).