A little bit of genetics 101 short hand:
P generation is the parental generation. (This is the stage I am at right now, selecting my P gen.)
F1 is first filial generation, the offspring of the P gen.
F2 is second filial generation, the offspring of at least one F1 parent.
F3, F4, etc continue. This shorthand is mostly associated with Mendelian genetics and the famous Punnet square. Works great with autosomal dominant/recessive ... kinda works with incomplete or co-dominant (autosomal, again) and goes flying out of the window with sex-linked genes. I enjoyed my intro to genetics semester.
P gen does not necessarily need to be purebred, nor do they need to be different breeds/varieties, they are just the parental generation you start with. In my case, P gen will composed of hatchery GLWs and black phase BLR from Luanne, which is technically a variety cross although both will be Wyandotte.