MTBF usually stands for "mean time between failures" and is defined as the average time between failures of a given system, according to EventHelix.com. This is calculated by taking the sum of all downtime and dividing by the number of system failures.
MTBF is the average time a manufacturer predicts before a failure in occurs in a system. Wikipedia reports that the value of a product's MTBF is an important element in its development. Engineers calculate a product's MTBF using specialized software according to different standards. MTBF figures reported by manufacturers are often very different than what is encountered in the field. Digi.com reports some consumers who regard it as a "meaningless" number.