MTBF is the Mean Time Between Failures of a piece of hardware. For hard disk drives it is on the order of 300,000 hours (about 34 years!) and is measured by testing a very large number of hard disk drives for a few hours and seeing how many fail. It isn't a very good measure of the life expectancy of computer hardware, because it only takes account of random failures, not those caused by the disk drive wearing out, which happens after 3-5 years on average.