A few days ago, I remember one of our team mates to have asked on the concept of Bell Curve and here is my brief answer to the same..
A bell curve is a theoretical dsitribution of scores across a given population. Applied to PMS, this means that the performance of the majority of a given population will tend to gravitate towards the center i.e., average while the extremes would comprise of either the poor performers or excellent ones. This simply means that you can't have 100% stars in a team; there has to be the so-so (average) guys and low performers. The curve is mostly used as a control mechanism.
Hope it helps ; )