Great questions! F1 means filial 1 and it's the first generation from a hybrid cross. Most of the corn you eat, for instance, is f1 because those crosses result in something called hybrid vigor. They grow bigger and stronger. This happens only when two very inbred lines are crossed together. It's pretty amazing, you can take two crap strains and produce amazing pants by crossing them. In fact, you would not want to grow or eat the parents of the corn that we do grow or eat.
Us cannabis pollen chuckers aren't very bright though, so we use the term f1 to refer to any cross between two strains. F2 would be if you then took the children of this cross and crossed them
S1 means self generation 1, it is when you cross a plant with itself. You take a female clone, use chemicals to make it turn make, and then use that pollen on the female. Actually now that I think about it I don't know why we put a number on it because how would s2 even be possible if you're selling? As I said, we're not very bright. 😆😆