Population is increased by 1/10th of the reproduction rate every X turns, where X is that frequency. Thus, in stock, you sort of get the reproduction rate being growth in a year. It is yearly growth, compounded monthly.
If you set Repr. Freq. to 2, it means no pop will grow after one turn, and you will get 1/10th repro rate growth on the second turn. Set it to 3, you get the 1/10th growth every 3 turns, and so on.