RealPower is an attempt to calculate the effective power of multi-cpu computers. It is a rough but useful guide number. A dual-CPU 2GHz Core 2 has a RealPower number of 3.2 which means that it would perform similarly to a theoretical single 3.2GHz Core 2 CPU. RealPower is no replacement for real benchmarks with the applications you use.
Sorting by CPU in the Hardware List window uses the RealPower factor to estimate and sort the computers according to their likely performance. You can, for example, search for all free and working computers and then sort them by RealPower (click the CPU header) to quickly find the best candidates for a new employee.
The RealPower value is also displayed in the Hardware Details window, along with some Geekbench stats.
The actual formula that is used (in Excel syntax) =POWER(1.6,LOG(CPUs,2))