MPBR is the range at which the bullet drops x inches below line of sight after having risen no more than that same x inches above the line of sight. According to the Oehler Ballistic Explorer my .30-06’s favorite factory load, the Federal High Energy with the Nosler Partition 180 grain is dead on at 26 yards, 3" high just beyond 140 yards, dead on again at 244 yards, and 3" low at 286.2 yards. The idea is that out to 286 yards range will not produce more than a 3" elevation error. This is for a 6" vital zone.
Of course the Ballistic Explorer, and most other programs, works from a manufacturer’s muzzle velocity and ballistic coefficient, so the calculated MPBR is only a good starting point for testing your individual rifle.