S is short for selfed; the 1 is what generation it is. What you do is light stress a female plant (3 day periods switching from 24/0 to 12/12) to induce it into throwing everything it has into flowering. This will determine whether the plant is a hermie or true female (you are supposed to use a true female). There are recipes for home science project type ways of making what's called colloidal silver (people sell it online too). When you spray this substance on the nodes where the true female parts normally form, the parts produced by the plant come out male. They call it selfing because you're crossing it with its own chromosome. Usually the plant will produce its own seeds but I have a clone and a sister S1 plant in with it; I don't see a single female flower on the whole treated plant, but it probably has a few seeds buried in the male flowers.