Suppose an experiment set out to sample the incomes of adults. Obviously, the incomes of college graduates might differ from that of non-graduates. Now suppose the number of male graduates was 30% of the total male adults (imaginary figures). Then you would arrange for 30% of the total sample to be male graduates picked at random, and 70% of the total to be male non-graduates. Repeat the process for females, because the percentage of female graduates is different from males. That gives a sample of the adult population stratified by sex and college education. The next step would be to divide each of your sub-populations by age groups, because (for example) graduates might gain more income relative to non-graduates in middle age.