Adjusting phenotypes by regressing out covariates
Instead of the "observed phenotype", it's asking for a "corrected phenotype", which is the residuals from pheno ~ covariates. In R, one would get that as follows:

fit = lm(phenotype ~ covariates, data)
corrected_phenotype = resid(fit)


Of course covariates is just a place hold for something like age + gender or something along those lines.