Raw Physical Power.
This is a figure arrived at by multiplying one's weight at one standard gravity by his or her gravity rank.
Dark RPP is the above figure multiplied further by the Artifact rank.
Comparing RPPs alone is not the definitive method for determining who would be successful in a contest. Technique will alter the numbers--sometimes significantly--and other factors such as speed, skill, experience, and environmental conditions will definitely play a part, but the RPP does provide a baseline from which real comparisons become possible.