There should be an explanation in the patternnotes before the cast on, or somewhere else in the book.
SskS2KP = slip slip ktog, slip 2, k psso. Maybe there should be a comma, like this - ssk, s2kp. It decs 3 sts but I think it's supposed to result in one stitch somehow.
S2K2togP = this is easier, slip 2 as if to knit tog, k2tog, pass the slip sts over. This would be a triple dec.