Mean Time Between Failure.
Most likely it is measured by testing the particular product for "x" length of time then measureing the wear level, then using that to predict how long before total wear of failure. I'm sure that is not the exact answer, but it is probably close. You'd have to ask a particular manufacturer questions on specific items for more specific answers.
As to the MTBF on MB's?? I'm not sure that there is one. Mostly MTBF is for something mechanical in nature with moving parts; like fans or PS's.