MTBF = Mean Time Between Failure
It is a estimate of How many hours a electronic system can operate before a failure will occur. Generally a real wag.
Ie 1 Million Hrs = like about 114 years running 24 hrs/day.
Mean Time Before Failure is measure in a very strange way that makes it nearly worthless (to a consumer). Essentially, it is measured by testing several devices together; when the first device fails, the test is over, and the MTBF is calculated by:
MTBF = (number of devices being tested) * (time to first failure);
(edit: for what it's worth, I feel like I've got something a little wrong here, but I'm not quite sure what. This is at least quite close to how the measurement is performed )
So, in theory they could have run 1 million devices and had one device fail at the 1 hour mark.
Further, there is nothing official about this number, they can pretest the devices they use, set the test conditions, and retest as much as they want.
It is essentially a useful engineering metric, that can be twisted into a nice looking marketing number that makes it sound like the device you are buying will never fail.