Understanding S.M.A.R.T data

[Daniel3628]

Worst and Current seem pretty self-explanatory. It's shows the worst and the current speed (I assume in miliseconds) my hard drive has ever done, but what does Real Value and Threshold mean?

 

I would think Threshold means when this number is reached, it's time to replace the hard drive, but no. If you look at the picture below, you will see my hard drive is in GOOD health even though the current and worst numbers are about 4 times greater than the Threshold number on ID 1, 6x on ID 2, 1.5x on ID 5, and an unmeasureable amount higher on the rest of them.