Just don't really understand what these are refering too,

what is the F in F1 or F2 and also S1 S2

f1 hybrid is a term used in breeding. look up "breeding marijuana".

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f1 hybrid is term used in genetic breeding.

Here is a link to how the genetic classification works.
It's VERY specific, and the terms f1, f2, s1, s2 get
thrown around a lot by people who do not understand
how genetic breed works.

Cannabis or not... this link will help explain things better:
http://anthro.palomar.edu/mendel/mendel_1.htm

Good luck.

EDIT: Only pay for f1 hybrids. Everything else is no different
than any bag seed you find, because you only get a 25% chance
of getting the genetics you want from a f2 hybrid. The other
75% could be unwanted, unstable, genetic traits and phenotypes.