that number is how many millimeters the center of the BB is lower than the center of the rear axle. The reason they use this number more often than BB height is because its the same no matter the wheel size. If you want to find BB height, take half of your wheel size, and subtract the BB drop, and that will give you the height of it off of the ground.
assuming your on 700's:

622mm/2 - 50mm= 261mm = 10.3inches
622mm/2 - 58mm= 253mm = 9.96inches
this i without a tire mounted.

/notscrod