Effectively its indicating how many sectors within your hard drive have been substituted for a reserve sector. It's usually an indicator of a failing hard drive if it has to reallocate sectors, as the sector that has been 'reallocated' is no longer visible to your operating system as somewhere, somehow, it has either become corrupt or damaged, so your HDD removes it, so that it doesn't cause any problems.
So 200 is the normal value, and if 200 falls below 140, then crystal disk will identify it as a failure. So effectively, the RAW VALUE, tells you how many sectors have been reallocated. If a sector is reallocated, your current will 'effectively' move down to '199', which means a sector has become corrupt, and your raw value will go up.
if this 200 falls below the THRESHOLD, then it indicates a HDD failure. 200, is the normal value might I add!