Simply put MTBF (Mean time between failure) is the number of hours the product is rated for before you could expect a hardware failure. See link below for a more detailed definition of MTBF
http://en.wikipedia.org/wiki/Mean_time_between_failures
That said SSD life is highly dependent on how heavy of a write load you have in your environment. A typical consumer usage probably would not come close to what these drives can handle within the warranty period. Best thing to do is keep a check on your SMART attributes. If you see one of the attributes trip to fail it is time to replace the drive.
