Think of it as a positional replacement level proxy. It's easiest to think about it with catchers. Let's say we're talking 5x5 ottoneu. If Buster Posey has 75 runs and 75 RBI, those are pretty bland numbers for a first baseman. At catcher, they may well lead the position.
The calculator is getting values for runs and RBI relative to ALL players. However, Posey will actually net your team many runs and RBI relative to all catchers. So a position adjustment is needed to properly account for that.