When exercising you may wonder, "how hard do I need to exercise to improve my health?". Then you may wonder "well...how hard do I have to exercise to work off those cookies from yesterday?". Either way there is a way to gauge your effort instead of just competing with the person on the machine next to you.
The BORG scale is a measurement of perceived exertion, which means the scale is based on YOU. Not the personal trainer whose biceps are so big he can't reach into his pockets. The BORG exertion scale allows you to determine how hard you are working depending on how hard you have pushed in the past and how fit you are at that time.
In general, if you can exercise in the 4-6 moderate zone then you will get the optimum cardio-respiratory benefits. Of course if you have injuries or health issues then stay in more of the 2-3 light zone. Therefore there is no excuse to not be active!
P.S. For reference, the participants on the Biggest Loser are probably operating more at 9-10 very hard zone... fainting and puking are not a badge of honor!