They are testing whether personality and IQ are correlated to success. R-squared is a measurement of dependence. So basically they are graphing this correlation

They are testing whether personality and IQ are correlated to success. R-squared is a measurement of dependence. So basically they are graphing this correlation on a scale from from 0 being "this variable is independent of success", to 1 being "success is perfectly correlated with this variable". The actual measurements are in a separate appendix, so I'm not sure how the calculated all this.