MTBF is a measure of the reliability of units. The higher the MTBF value, the more "reliable" the unit is.
Average operating time between failures: The operating time means the operating time between two successive failures of a unit to be repaired.
For units that are not repaired, the mean time to failure (MTTF) is the expected value (average) of the distribution of lifetimes. Colloquially, the terms are often used synonymously.
Under unfavorable operating conditions, significantly lower MTBF values (higher failure rates) can occur. On the other hand, derating can cause the MTBF to be higher (failure rate lower).
MTBF stands for Mean Time Between Failures. The MTBF indicates when, on statistical average, a device fails due to processing or component faults before signs of wear occur.