In order to understand MTBF (Mean Time Between Failures) it is best to start with something else -- something for which it is easier to develop an intuitive feel. This other concept is failure rate which is, not surprisingly, the average (mean) rate at which things fail. A "thing" could be a component, a signal conditioner, or a whole system. Some things -- rocks, for example -- are accepted to have very low failure rates while others -- British sports cars, for example -- are expected to have relatively high failure rates.

It is generally accepted among reliability specialists that a thing's failure rate isn't constant, but generally goes through three phases over a thing's lifetime. In the first phase the failure rate is relatively high, but decreases over time, this is called the "infant mortality" phase. In the second phase the failure rate is low and essentially constant, this is called the "constant failure rate" phase. In the third phase the failure rate begins increasing again, often quite rapidly, this is called the "wearout" phase. The reliability specialists noticed that when plotted as a function of time the failure rate resembled a familiar bathroom appliance, they called it a "bathtub" curve. The units of failure rate are failures per unit of "thing-time"; e.g. failures per machine-hour. Signal conditioners, being electronic, are usually measured in "power on hours" (POH).

What, you may ask, does all this have to do with MTBF?  Read more.