Folding power controls how much of your computer's resources Folding@home uses.
If you want to see what your high performance system can do crank it up to Full.
If your computer starts to stress out move it to Light to give it a break.
These settings have the following meaning:
Light 20%
Medium 60%
Full 100%
Actual usage will be less than these values and depends on how many CPU cores your system has available and what other processes are running on your computer at the time.