Its exactly what it sounds like, the time to spin up the motor on your hard drive (from 0 to 5400, 7200, 10000, or 15000 rpm, whichever your drive is). Since the only time the drive isn't running at its max speed is when its off and immediately after you wake it up or power on it isn't affecting performance. If it 'passed' everything else I wouldn't be too concerned about it.
If you are still concerned, do a full diagnostic test on it with the drive tools supplied by the drive manufacturer.