MTBF (Mean Time Between Failure) is the probability of a "random equipment failure".
Various standards are available to calculate the MTBF values; for instance SN 29500, IEC 61709, MIL HDBK 217F and Belcore amongst others. The calculation is always made in the same way.
The standard provides a database where base failure rates are available for individual components which are then adjusted with the stress factor for the actual application. The calculated failure rates of all components are added up and result in the failure rate for the overall unit.
The MTBF number indicates the statistical probability of failure. An MTBF figure of 1,000,000 means if there are 1,000 units in service then statistically one unit will fail every 1,000 hours. However, it is not possible to state whether a failed device was already in service for 50,000 or only 100 hours.
MTBF hours must not be confused with service life hours. PULS gives detailed values for both properties in the product data sheets.